Tag Archives: sql server

Using DataSets to work with (relational) in-memory data

After a previous blog, Using C# to connect to and query from a SQL database, I got a request to write a bit more about the SqlDataAdapter and DataSets. So here it goes.

Looking at the DataSet

So what exactly is a DataSet? Besides it obviously being a class in the .NET Framework the DataSet represents an in-memory cache of data. This data can be stored in multiple DataTables. DataTables contains DataRows, which are made up of DataColumns. DataTables in a single DataSet can be related to each other by what can best be described as foreign key relations, the DataRelation is used to establish such a relation. Additionally DataTables can contain contraints such as UniqueContraints and ForeignKeyContraints. In case you hadn’t noticed yet, this is the relational model we know from SQL databases!
So yes, you’d think that working with DataSets and SQL kind of go hand-in-hand and it is in fact quite easy to fill a DataSet using the SqlDataAdapter. We’ve seen this in the previous blog post, but let’s quickly take a look at it again.

DataTable table = new DataTable();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person", connection))
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
{
    adapter.Fill(table);
}

Notice that I’ve used a DataTable instead of a DataSet. In this case all I’m getting from the database is a single resultset. If I would’ve used a DataSet the DataAdapter would’ve filled it with one DataTable, the one we have now. So in the next example I’m going to use a DataSet and do something with the data we get.

DataSet set = new DataSet();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person", connection))
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
{
    adapter.Fill(set);
}

foreach (DataTable table in set.Tables)
{
    foreach (DataRow row in table.Rows)
    {
        Console.WriteLine("ID: {0}, Name: {1} {2}.", row["ID"], row["FirstName"], row["LastName"]);
    }
}

Console.ReadKey();

Notice that I loop through the tables of the DataSet and through the Rows of the DataTable. the row["ColumnName"] accesses the value (boxed in an object) of the column in that row. Obviously if I had two tables, the other not being a Person, this code would break.

Accessing multiple tables and schema’s

So in the next example we’re going to load multiple tables into our DataSet. By default, the DataSet creates tables with the name “Table”, “Table1”, “Table2”, etc. If we want to access our tables by name rather than index, like we do with our columns, we’ll have to name them ourselves. You’ll see this in the next example.

DataSet set = new DataSet();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID FROM Person.BusinessEntityrn" +
    "SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person", connection))
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
{
    adapter.Fill(set);
    set.Tables[0].TableName = "BusinessEntity";
    set.Tables[1].TableName = "Person";
}

foreach (DataTable table in set.Tables)
{
    if (table.TableName == "Person")
    {
        foreach (DataRow row in table.Rows)
        {
            Console.WriteLine(String.Format("ID: {0}, Name: {1} {2}.", row["ID"], row["FirstName"], row["LastName"]));
        }
    }
    else if (table.TableName == "BusinessEntity")
    { //... 
    }
}

Console.ReadKey();

And even better would be to not loop through tables at all, but simply access the table you want by name.

DataTable personTable = set.Tables["Person"];
foreach (DataRow row in personTable.Rows)
{ //...
}

As you can see we’re querying for two tables (the first only having the ID column) and the second being our Person. It’s also possible to create your DataSet without having to query for data. This can be done by using the FillSchema method of the DataAdapter. The following will result in the same tables we had in the previous example, but without data.

DataSet set = new DataSet();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID FROM Person.BusinessEntityrn" +
    "SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person", connection))
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
{
    adapter.FillSchema(set, SchemaType.Source);
    set.Tables[0].TableName = "BusinessEntity";
    set.Tables[1].TableName = "Person";
}

Of course the SqlTableAdapter still has to do a roundtrip to the database to get the schema information. It will send the following query to get the schema (I’m not sure if it’s the same for all versions of .NET and/or SQL Server).

SET FMTONLY OFF; SET NO_BROWSETABLE ON; SET FMTONLY ON;SELECT BusinessEntityID AS ID FROM Person.BusinessEntity
SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person
SELECT BusinessEntityID AS ID, AddressID, AddressTypeID FROM Person.BusinessEntityAddress
SELECT AddressID, AddressLine1, AddressLine2, City, PostalCode FROM Person.Address SET FMTONLY OFF; SET NO_BROWSETABLE OFF;

Getting relational

I mentioned that DataSets can also have relations between tables. Unfortunately you’ll have to add those yourselves. Why would you want to do this? Because you can now navigate from a row in one table to child rows in other tables. BusinessEntity and Person are related, one BusinessEntity represents one Person. However, a BusinessEntity can have one or more addresses through the BusinessEntityAddress table which has a relation to Address. Let’s load all those tables into our DataSet, create the appropriate relations and display the names and addresses of our business entities (note that I’m only printing the ID’s and names of people who have at least one address). Remember that in order to create a relation all rows from a child table need a row in their master table (just like in the database). This is especially tricky when you start using filters. For example getting ONLY Persons with FirstName like ‘A%’, but getting ALL addresses will result in addresses without a person (or a child without a master), which prevents you from creating a relation.

DataSet set = new DataSet();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID FROM Person.BusinessEntityrn" +
    "SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Personrn" +
    "SELECT BusinessEntityID AS ID, AddressID, AddressTypeID FROM Person.BusinessEntityAddressrn" +
    "SELECT AddressID, AddressLine1, AddressLine2, City, PostalCode FROM Person.Address", connection))
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
{
    adapter.Fill(set);
    set.Tables[0].TableName = "BusinessEntity";
    set.Tables[1].TableName = "Person";
    set.Tables[2].TableName = "BusinessEntityAddress";
    set.Tables[3].TableName = "Address";

    set.Relations.Add("BusinessEntity_Person", set.Tables["BusinessEntity"].Columns["ID"], set.Tables["Person"].Columns["ID"]);
    set.Relations.Add("BusinessEntity_BusinessEntityAddress", set.Tables["BusinessEntity"].Columns["ID"], set.Tables["BusinessEntityAddress"].Columns["ID"]);
    set.Relations.Add("BusinessEntityAddress_Address", set.Tables["BusinessEntityAddress"].Columns["AddressID"], set.Tables["Address"].Columns["AddressID"]);
}

foreach (DataRow entityRow in set.Tables["Businessentity"].Rows)
{
    foreach (DataRow personRow in entityRow.GetChildRows("BusinessEntity_Person"))
    {
        foreach (DataRow entityAddressRow in entityRow.GetChildRows("BusinessEntity_BusinessEntityAddress"))
        {
            foreach (DataRow addressRow in entityAddressRow.GetChildRows("BusinessEntityAddress_Address"))
            {
                Console.WriteLine("The entity with ID {0} has name {1} {2} and address {3} {4} {5}.",
                    entityRow["ID"], personRow["FirstName"], personRow["LastName"],
                    addressRow["AddressLine1"], addressRow["City"], addressRow["PostalCode"]);
            }
        }
    }
}

Console.ReadKey();

Now that’s quite a bit of code… I’m using a lot of (magical) strings to illustrate how it works. I suggest you create some constants or something. Don’t duplicate strings like that.
So take a good look at that example and let it sink in. You’ve just loaded your database, including relations, in code! That’s pretty neat! But it gets even better!

CRUD operations using DataSets

So we’ve looked at reading data, but what about creating, updating and deleting data? To do this you need to set the Insert, Update and DeleteCommands of your DataAdapter and call the Update method. You can do this manually, simply create three SqlCommands, one with an INSERT statement, one with an UPDATE statement and one with a DELETE statement. You can do with less if you know that certain actions are impossible, for example records from a certain table could never be deleted.
You can also use the SqlCommandBuilder, like I will do in the next example. The SqlCommandBuilder can create the insert, update and delete commands for you based on your (single table!) select query. So make sure you use the same select query in both your actual select and your update routine! The SqlCommandBuilder needs to get the schema of your table and does this using your select query. That also means it needs to make a roundtrip to the database (the same we saw with FillSchema). So when you really need that performance, or if you want to do things ‘right’, set your commands manually.
Another caveat when updating records is that you can only update one table at a time and it will do so row by row. If you update a DataSet and don’t specify a table the Update method will use the first table in the DataSet.
Before updating you can call HasChanges (to prevent going to the database if you have no changes) and GetChanges, which returns a new DataSet containing only changed DataTables and DataRows. When you do you’ll need to explicitly call AcceptChanges on your original DataSet to indicate the changes were successfully saved to the data source. Alternatively you can call RejectChanges to restore the DataSet to its original state (since the last Accept- or RejectChanges was called or since it was created). You can also accept or reject changes on DataTable and even DataRow level.
That’s A LOT of information, let’s look at some code!

DataSet set = new DataSet();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person ORDER BY ID", connection))
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
{
    adapter.Fill(set);
}

set.Tables[0].Rows[0]["FirstName"] = "Sander";

if (set.HasChanges())
{
    DataSet changes = set.GetChanges();
    using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
    using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person ORDER BY ID", connection))
    using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
    using (SqlCommandBuilder builder = new SqlCommandBuilder(adapter))
    {
        adapter.Update(changes);
        set.AcceptChanges();
    }
}

You can insert and delete rows in the same manner. Let’s look at inserting a record. In this case we always need to insert a BusinessEntity or we’ll create a Person without a BusinessEntity. This is actually more complicated than it sounds. First, we’ll need all the columns that do not allow NULL values and set values. Second we need to let our DataSet know that there’s a relation between BusinessEntity and Person. We’ll also need to let the DataSet know that the BusinessEntityID is an ID field with an Identity Specification (which means we don’t have to set it ourselves), but our Person needs that same ID (we can use FillSchema for this!). Last we need to issue an update for each table seperately, which means creating seperate SqlCommands, SqlDataAdapters and SqlCommandbuilders…

DataSet set = new DataSet();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID FROM Person.BusinessEntityrn" +
    "SELECT BusinessEntityID AS ID, PersonType, NameStyle, FirstName, MiddleName, LastName, EmailPromotion FROM Person.Person", connection))
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
{
    adapter.TableMappings.Add("Person.BusinessEntity", "BusinessEntity");
    adapter.TableMappings.Add("Person.Person", "Person");
    adapter.FillSchema(set, SchemaType.Source);
    adapter.Fill(set);
    set.Tables[0].TableName = "BusinessEntity";
    set.Tables[1].TableName = "Person";
    set.Relations.Add("Relation", set.Tables["Businessentity"].Columns["ID"], set.Tables["Person"].Columns["ID"]);
}

DataTable businessEntityTable = set.Tables["BusinessEntity"];
DataRow newBusinessEntityRow = businessEntityTable.NewRow();
businessEntityTable.Rows.Add(newBusinessEntityRow);

DataTable personTable = set.Tables["Person"];
DataRow newPersonRow = personTable.NewRow();
newPersonRow["PersonType"] = "EM";
newPersonRow["NameStyle"] = 0;
newPersonRow["FirstName"] = "Sander";
newPersonRow["LastName"] = "Rossel";
newPersonRow["EmailPromotion"] = 0;
newPersonRow.SetParentRow(newBusinessEntityRow);
personTable.Rows.Add(newPersonRow);

if (set.HasChanges())
{
    DataSet changes = set.GetChanges();
    using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
    using (SqlCommand businessEntityCmd = new SqlCommand("SELECT BusinessEntityID AS ID FROM Person.BusinessEntity", connection))
    using (SqlDataAdapter businessEntityAdapter = new SqlDataAdapter(businessEntityCmd))
    using (SqlCommandBuilder businessEntityBuilder = new SqlCommandBuilder(businessEntityAdapter))
    using (SqlCommand personCmd = new SqlCommand(
        "SELECT BusinessEntityID AS ID, PersonType, NameStyle, FirstName, MiddleName, LastName, EmailPromotion FROM Person.Person", connection))
    using (SqlDataAdapter personAdapter = new SqlDataAdapter(personCmd))
    using (SqlCommandBuilder personBuilder = new SqlCommandBuilder(personAdapter))
    {
        businessEntityAdapter.Update(changes, "BusinessEntity");
        personAdapter.Update(changes, "Person");
        set.AcceptChanges();
    }
}

We can also delete rows. In this database it’s a bit difficult because everything is related and some entities simply cannot be deleted (only made inactive). But we can, of course, delete our just inserted row. Most of the code above remains the same, except the part where we created the DataRows. I’ll use some LINQ to retrieve the last inserted BusinessEntity (assuming it’s the row we just inserted). Once I got the row I’ll first delete the child rows (Persons) and then I’ll delete the BusinessEntity.
Now here comes the tricky part, in the example above we first needed to insert the BusinessEntity to generate an ID so we could insert the Person. For deletion it’s the other way around! That means that if you would do updates, inserts and deletes all in one you’d get an exception. Either your inserts fail because you insert child rows before their parent rows are inserted or you delete parent rows before their child rows are deleted.
You can use an overload on GetChanges that accepts a DataRowState to create two DataSets, one with updates and inserts and one with deletes and execute them in the correct order.
In the next example I’ll use the DataRowState, but I’m only deleting rows.

DataSet set = new DataSet();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID FROM Person.BusinessEntityrn" +
    "SELECT BusinessEntityID AS ID, PersonType, NameStyle, FirstName, MiddleName, LastName, EmailPromotion FROM Person.Person", connection))
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
{
    adapter.TableMappings.Add("Person.BusinessEntity", "BusinessEntity");
    adapter.TableMappings.Add("Person.Person", "Person");
    adapter.FillSchema(set, SchemaType.Source);
    adapter.Fill(set);
    set.Tables[0].TableName = "BusinessEntity";
    set.Tables[1].TableName = "Person";
    set.Relations.Add("Relation", set.Tables["Businessentity"].Columns["ID"], set.Tables["Person"].Columns["ID"]);
}

DataTable table = set.Tables["BusinessEntity"];
List rows = table.Rows.Cast().ToList();
int maxId = table.Rows.Cast().Max(b => b.Field("ID"));
DataRow row = rows.Single(be => be.Field("ID") == maxId);
foreach (DataRow personRow in row.GetChildRows("Relation"))
{
    row.Delete();
}
row.Delete();

if (set.HasChanges())
{
    DataSet changes = set.GetChanges(DataRowState.Deleted);
    using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
    using (SqlCommand businessEntityCmd = new SqlCommand("SELECT BusinessEntityID AS ID FROM Person.BusinessEntity", connection))
    using (SqlDataAdapter businessEntityAdapter = new SqlDataAdapter(businessEntityCmd))
    using (SqlCommandBuilder businessEntityBuilder = new SqlCommandBuilder(businessEntityAdapter))
    using (SqlCommand personCmd = new SqlCommand(
        "SELECT BusinessEntityID AS ID, PersonType, NameStyle, FirstName, MiddleName, LastName, EmailPromotion FROM Person.Person", connection))
    using (SqlDataAdapter personAdapter = new SqlDataAdapter(personCmd))
    using (SqlCommandBuilder personBuilder = new SqlCommandBuilder(personAdapter))
    {
        personAdapter.Update(changes, "Person");
        businessEntityAdapter.Update(changes, "BusinessEntity");
        set.AcceptChanges();
    }
}

And there you have it! No one said working with DataSets was easy… In this post we looked at DataSets and SqlDataAdapters, it is possible to use DataSets with other data sources such as XML though! In fact, DataSets and DataTables are unaware of their data source, you can fill them however you like. Their structure also makes them perfect for binding to DataGridViews in WinForms (and probably WPF too, but I wouldn’t really know).
Their weakness is that they are not strong-typed. We are forced to work with strings to refer to table and column names, and all values contained in them are objects (which means they could be anything).
Microsoft saw this and created so-called Typed DataSets. I recommend you skip Typed DataSets and move straight to LINQ-To-SQL or the Entity Framework. That’s a whole different topic though.

Comments are welcome. Happy coding!

Using C# to connect to and query from a SQL database

As a developer you’ll probably spend a lot of time getting data in and out of a database. Data is important in any organization and your job as a developer is to present that data to a user, have them add or edit that data and store it back to the database.

Yet I have found that many developers really have no clue how to work with a database! Many developers can get data out of databases, but do so in an unsafe way that may break your code and, worse, give hackers an opportunity to get direct access to your database! Others use an ORM like NHibernate, Entity Framework or LINQ To SQL, but have no idea what’s going on. In this blog post I will address these issues: how to setup a database connection, query for data in a secure manner and use that data in your code. I’ll also show you how to push data back to a database.

I am assuming you know how to set up a database and you know your way around C# and the .NET Framework. For my example I have used the Adventure Works 2014 Sample Database on a SQL Server 2014 database.

So let’s start. To create a connection to a database you’ll first need a database connection object. In our case we need a specific type of connection object, being the SqlConnection. Using the SqlConnection you can configure all kinds of settings that are used for your current session to the database. In this blog we’ll use defaults only. For creating the SqlConnection we’ll use the constructor that takes a (connection)string as input parameter. Usually you’d get the connectiongstring from a config file or some such. Alternatively you can create one using the SqlConnectionStringBuilder, but I won’t go into that here. Notice that I’ve wrapped the SqlConnection in a using block. This ensures that the connection is actually closed once we’re done with it. Make sure you actually open the connection only when needed.

using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
{
    connection.Open();
}

Unfortunately this doesn’t do anything yet. We’ll need a SqlCommand which takes the query we want to send to the database. In this case I’m going to select all persons from the table Person.Person. We can create a command object in different ways, but I’m going to create one using the constructor that takes the query and our just created connection. Once we have created the command we must open the connection (which we already did) and have it execute our query. There are a few ways to have the command actually execute your query.
The first is ExecuteNonQuery, which seems odd because we are going to execute a query, right? Well actually you use this method when you don’t expect a result (perhaps and update statement, or a call to a Stored Procedure that returns no resultset).
The second method, and the one we’ll need in this example, is ExecuteReader. This method returns a SqlDataReader which represents a forward-only stream of rows from the database. The columns of each row can be accessed by index or name. We’ll see how to use the SqlDataReader in the next example.
The third method, and last I will discuss, is ExecuteScalar. You can use this method when you expect exactly one result from a query.
There’s also an ExecuteXmlReader method which I will not discuss here. Additionally every method has its async versions. For older versions of .NET these are the BeginExecute and EndExecute methods and for later versions of .NET these are the ExecuteAsync methods. I will not discuss them here.
So let’s look at our example. We’re going to create a command to fetch some data from the Person.Person table and use ExecuteReader to get our results.

List persons = new List();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person", connection))
{
    connection.Open();
    using (SqlDataReader reader = cmd.ExecuteReader())
    {
        // Check is the reader has any rows at all before starting to read.
        if (reader.HasRows)
        {
            // Read advances to the next row.
            while (reader.Read())
            {
                Person p = new Person();
                // To avoid unexpected bugs access columns by name.
                p.ID = reader.GetInt32(reader.GetOrdinal("ID"));
                p.FirstName = reader.GetString(reader.GetOrdinal("FirstName"));
                int middleNameIndex = reader.GetOrdinal("MiddleName");
                // If a column is nullable always check for DBNull...
                if (!reader.IsDBNull(middleNameIndex))
                {
                    p.MiddleName = reader.GetString(middleNameIndex);
                }
                p.LastName = reader.GetString(reader.GetOrdinal("LastName"));
                persons.Add(p);
            }
        }
    }
}
// Use persons here...

You may have noticed that getting a value from a SqlDataReader isn’t easy! There are methods like GetString, GetInt32, GetBoolean, etc. to convert values from their database representation to their CLR type equivalents. Unfortunately they throw on DBNull values. So in case of MiddleName, which is a NULLABLE column in the database, we need to check for DBNull before setting the MiddleName value. In case of integer or booleans (or any non-nullable type) we would use the nullable equivalents of those types like int? or bool? (which is short for Nullable<T>).

Another method to get data from the database is by using a SqlDataAdapter. This results in a DataTable or DataSet (for multiple resultsets) containing the database data. I won’t go into the use of DataTables and DataSets, but they are like in-memory GridViews. They even track if a row was changed and can automatically generate update, insert or delete commands when used with a SqlCommandBuilder.
The next code snippet shows how to fill a DataTable (that’s a lot less code than the SqlDataReader example, but keep in mind that the result is also very different).

DataTable table = new DataTable();
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person", connection))
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
{
    adapter.Fill(table);
}
// Use table here...

For the next example we are going to select a subset of persons by first name. That means we’ll have to change our query. Let’s look at an example.

string firstName = "John";
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person WHERE FirstName = '" + firstName + "'", connection))
{
    // ...
}

Looking good, right? NO! THIS IS REALLY VERY WRONG! For John this works great (I’ll tell you in a moment why it works, but still isn’t great), but for D’Artagnan (a musketeer) this won’t work at all! While the apostrophe is all good in C# it ends a string in SQL. So the query you’ll be sending to SQL is SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person WHERE FirstName = D'Artgnan. Go to SQL server Management Studio, open a new query window and try to run that exact query. You’ll get an error message saying something about an unclosed quotation mark. What it should’ve been was D”Artagnan. But even replacing every apostrophe with double apostrophe won’t work.

Whenever you send a query to SQL Server a query plan is made and the fastest way to get your data is calculated. For our query SQL Server might decide it will use an index we placed on FirstName. Once the plan is decided it’s cached and re-used when the exact same query is called. In our example that would mean a plan is made and cached for each name we look for! That’s not very efficient since every plan will probably be the same anyway…

What’s even worse and THIS IS VERY IMPORTANT is that by concatenating strings to form a query like that is a HUGE SAFETY RISK! Maybe you’ve heard of SQL Injection Attacks. Let me demonstrate this. Let’s assume for a moment that the user gets a textbox to enter a name and that name is concatenated to your query like above. Now the user enters John'; USE master; DROP DATABASE AdventureWorks2014 -- and BAM! There goes your database… Really, it’s gone. I hope you have a backup. This technique is used to get personal information of users like email addresses and passwords.
Here is a mandatory xkcd on the subject:

xkcd: Exploits of a Mom

So how are we going to solve these problems? Parameterisation! By creating parameterized queries the query plan can be re-used for different values and SQL injection belongs to the past! So how does this look?

string firstName = "John";
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("SELECT BusinessEntityID AS ID, FirstName, MiddleName, LastName FROM Person.Person WHERE FirstName = @FirstName", connection))
{
    cmd.Parameters.AddWithValue("FirstName", firstName);
    connection.Open();
    using (var reader = cmd.ExecuteReader())
    {
        // ...
    }
}

And that’s how easy it is! Notice that by adding a parameter we also improved the readability of our code. Wow, that’s a win-win-win situation!
There is one caveat though, when you want to pass a NULL to the database you’ll have to use the DBNull.Value object instead of simply null. So when fetching data we converted DBNull to null and now we’ll have to convert null to DBNull. We’ll see this happening in the next example.

Now what if we want to update, insert or delete a record in the database? We can go about it in much the same way, but use ExecuteNonQuery (which returns the number of affected rows only).

int businessEntityID = 1;
string firstName = "Sander";
string middleName = null;
string lastName = "Rossel";
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("UPDATE Person.Person SET FirstName = @FirstName, MiddleName = @MiddleName, LastName = @LastName WHERE BusinessEntityID = @BusinessEntityID", connection))
{
    cmd.Parameters.AddWithValue("FirstName", firstName);
    if (middleName == null)
    {
        cmd.Parameters.AddWithValue("MiddleName", DBNull.Value);
    }
    else
    {
        cmd.Parameters.AddWithValue("MiddleName", middleName);
    }
    cmd.Parameters.AddWithValue("LastName", lastName);
    cmd.Parameters.AddWithValue("BusinessEntityID", businessEntityID);
    connection.Open();
    cmd.ExecuteNonQuery();
}

I have to add that it’s generally a good idea to check for null for ALL your parameters. You can make a helper function to prevent your code from cluttering up to much.
And in case you want your original Person back, here are his first-, middle- and last name: Ken J Sánchez.

So far we have only worked with plain text queries. Many times you’ll want to execute a stored procedure. This works in much the same way as sending your query to the database. You simply have to set the CommandType of your command to StoredProcedure and pass in the parameters.

int businessEntityID = 1;
string nationalIDNumber = "295847284";
DateTime birthDate = new DateTime(1987, 11, 8);
char maritalStatus = 'S';
char gender = 'M';
using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=AdventureWorks2014;Integrated Security=SSPI"))
using (SqlCommand cmd = new SqlCommand("HumanResources.uspUpdateEmployeePersonalInfo", connection))
{
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("BusinessEntityID", businessEntityID);
    cmd.Parameters.AddWithValue("NationalIDNumber", nationalIDNumber);
    cmd.Parameters.AddWithValue("BirthDate", birthDate);
    cmd.Parameters.AddWithValue("MaritalStatus", maritalStatus);
    cmd.Parameters.AddWithValue("Gender", gender);
    connection.Open();
    cmd.ExecuteNonQuery();
}

In case you want your original employee back, here is his original birthdate: 1969-01-29.

Perhaps you have noticed that the SqlConnection inherits from DbConnection which implements IDbConnection. We have also used other classes like the SqlCommand and SqlDataReader which inherit from DbCommand and DbDataReader in a same manner. The only thing you need to know right now is that many database providers have these classes as a common base class which means that if you know how to connect to SQL Server you (more or less) know how to connect to most SQL databases like Oracle, MySQL, PostgreSQL, Firebird, etc. In theory (and probably in practice too, although I’ve never tried) you can create a flexible data layer that can switch seamlessly between (SQL) databases because of these common base classes and interfaces.

Well, there you have it. We have successfully and correctly selected data, updated data and executed a stored procedure using C#. I assume you can now guess how to use ExecuteScalar, which I mentioned, but haven’t discussed further. Things don’t stop here though. There’s much more like queries that return multiple result sets, stored procedures that return output parameters, BLOB’s, bulk operations, transactions… Way to much to discuss here. Luckily there are many books, articles and blogs on the subject.

Happy coding!