SqlDataClient not working with using statement but works when including the full namespace

I am converting a project from VB to C# and I ran into an issue with SqlClient. My classes make extensive use SqlConnection and SqlCommand. To start the conversion I created a new class library and slowly re-created all of my files in c#. I kept running into a red squiggle under SqlClient so I added it with a using statement at the top of the document, using System.Data.SqlClient; and... it didnt do anything. I still had a huge number of the following errors...

The type of namespace name 'SqlClient' could not be found (are you missing a using directive or an assembly referece?)

I clicked the lightbulb and Visual Studio recommended I replace SqlClient.SqlConnection conn = null with System.Data.SqlClient.SqlConnection conn = null. After replacing it it actually turned grey. I moused over it again to find

Name can be Simplified

When I select that option it deletes the entire System.Data.SqlClient string from the call and leaves me with SqlConnection conn = null

So it looks like it looks like using System.Data.SqlClient allows me to directly access SqlConnection and other methods but it wont let me reference SqlClient. I already have using System.Data; and when I type "Sql" the only autocomplete suggestion is "SqlDbType"

Where in the world is SqlClient?

Answers


In VB.NET, you can "import" a parent namespace and reference variables using the inner namespace:

Foo.vb:

Namespace Outer
    Namespace Inner
        Public Class Foo
        End Class
    End Namespace
End Namespace

Program.vb:

Imports Outer

Public Class Program
    Dim f As Inner.Foo
End Class

However, in C#, you can't do this - you have to either put a using at the top with the full namespace, or fully qualify the type in the code:

using Outer.Inner;

public class Program {
    Foo foo;
}

or

public class Program {
    Outer.Inner.Foo foo;
}

The exception in C# would be if you are in your own namespace, you can use relative namespaces:

Foo.cs

namespace Outer {
    namespace Inner {
        public class Foo { }
    }
}

Program.cs

namespace Outer { // Or this could even be Outer.SomethingElse
    public class Program {
        Inner.Foo foo;
    }
}

System.Data.SqlClient is a namespace, not an actual type. You can add the using System.Data.SqlClient; statement at the top of your code and then reference the public (exported) types that are grouped in that namespace directly without having to prefix the namespace to the type. In a nutshell namespaces are a way to organize/group types to avoid name collisions and allow for type organization.

Example1 - using statement

using System.Data.SqlClient;

namespace MyNamespace {
    public class MyClass {
        public void TestMethod(){
            SqlConnection myConnection = null; // no need for a namespace prefix
            System.Data.SqlClient.SqlConnection myConnection2 = null; // this is ok too, the namespace is redundant but that is fine
        }
    }
}

Example 2 - no using statement

// using System.Data.SqlClient; // no using statement

namespace MyNamespace {
    public class MyClass {
        public void TestMethod(){
            // SqlConnection myConnection = null; // this line would not compile if you remove the comment because the compiler cannot find a global type SqlConnection 
            System.Data.SqlClient.SqlConnection myConnection2 = null; // you need a namespace because there is no using statement at the top so no shortcuts
        }
    }
}

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