How to use SQL user defined functions in .NET?

Question

I created a scalar function in the DB

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER FUNCTION [dbo].[fn_GetUserId_Username]
    (
    @Username varchar(32)
    )
RETURNS int
AS
    BEGIN
    DECLARE @UserId int
    SELECT @UserId = UserId FROM [User] WHERE Username = @Username
    RETURN @UserId
    END

Now I want to run it within my .NET C# or VB.NET code.

I use Entity Framework, I tried to map it with function mapping and I did not success. i don't care to do it with simple DbCommand, the problem is that I get no results (the function exists in the Entities class):

public int GetUserIdByUsername(string username)
{
    EntityConnection connection = (EntityConnection)Connection;            
    DbCommand com = connection.StoreConnection.CreateCommand();
    com.CommandText = "fn_GetUserId_Username";
    com.CommandType = CommandType.StoredProcedure;
    com.Parameters.Add(new SqlParameter("Username", username));
    if (com.Connection.State == ConnectionState.Closed) com.Connection.Open();
    try
    {
        var result = com.ExecuteScalar(); //always null
    }
    catch (Exception e)
    { 
    }
    return result;
}

Is there any solution? Posts in either C# or VB.NET will be welcommed.

Answer 1

It sounds like the right way in this case is to use the functionality of the entity framework to define a .NET function and map that to your UDF, but I think I see why you don't get the result you expect when you use ADO.NET to do it -- you're telling it you're calling a stored procedure, but you're really calling a function.

Try this:

public int GetUserIdByUsername(string username)
{
    EntityConnection connection = (EntityConnection)Connection;            
    DbCommand com = connection.StoreConnection.CreateCommand();
    com.CommandText = "select dbo.fn_GetUserId_Username(@Username)";
    com.CommandType = CommandType.Text;
    com.Parameters.Add(new SqlParameter("@Username", username));
    if (com.Connection.State == ConnectionState.Closed) com.Connection.Open();
    try
    {
        var result = com.ExecuteScalar(); // should properly get your value
        return (int)result;
    }
    catch (Exception e)
    {
        // either put some exception-handling code here or remove the catch 
        //   block and let the exception bubble out 
    }
}

Answer 2

This is very similar to the above answer, but the below code allows you to call a UDF with any number of parameters and any return type. This might be useful as a more general solution. This also hasn't been tested thoroughly...I think it will have some problems with varchars.

internal static T1 CallUDF<T1>(string strUDFName, params SqlParameter[] aspParameters)
{
    using (SqlConnection scnConnection = GetConnection())
    using (SqlCommand scmdCommand = new SqlCommand(strUDFName, scnConnection))
    {
        scmdCommand.CommandType = CommandType.StoredProcedure;

        scmdCommand.Parameters.Add("@ReturnValue", TypeToSqlDbType<T1>()).Direction = ParameterDirection.ReturnValue;
        scmdCommand.Parameters.AddRange(aspParameters);

        scmdCommand.ExecuteScalar();

        return (T1) scmdCommand.Parameters["@ReturnValue"].Value;
    }
}

private static SqlDbType TypeToSqlDbType<T1>()
{
    if (typeof(T1) == typeof(bool))
    {
        return SqlDbType.Bit;
    }
         .
         .
         .
    else
    {
        throw new ArgumentException("No mapping from type T1 to a SQL data type defined.");
    }
}