How do you specify 'DEFAULT' as a SQL parameter value in ADO.NET?

I have a parameterized SQL query targetted for SQL2005 which is dynamically created in code, so I used the ADO.NET SqlParameter class to add sql parameters to SqlCommand.

In the aforementioned SQL I select from a Table Valued Function with has defaults. I want my dynamic sql to sometimes specify a value for these default parameters, and other times I want to specify that the SQL DEFAULT - as defined in the Table Valued Function - should be used.

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To keep the code clean I didn't want to dynamically add the SQL DEFAULT keyword and parameterize it when a non-default is to be used, I just wanted to set DEFAULT as the value of my SQLParameter.

Can I? What is best practice in such an instance?

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SQL query parameters take the place of literal values only.

You can't send an SQL keyword as the value of a parameter, just as you cannot send a table identifier, column identifier, list of values (e.g. for an IN predicate), or an expression. The value of the parameter is always interpreted as a literal value, as if you had included a quoted string literal or a numeric literal in your query.

Sorry, but you have to include an SQL keyword as part of the SQL query before you prepare that query.

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AFAIK, the only way to tell SQL Server to use a default value is via the DEFAULT keyword or to exclude it from parameter list. That means that the use of the DEFAULT keyword must be in your parameterized SQL Statement. So, something like:

Select ...
From dbo.udf_Foo( DEFAULT, @Param2, @Param3, DEFAULT, .... )

I suppose another approach would be to query the system catalogs for the actual value of the various DEFAULT values and determine whether to set the SqlParameter to the default value that way, but that requires a convoluted second query to get the default values.

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If you have the following function (for example):

CREATE FUNCTION dbo.UFN_SAMPLE_FUNCTION 
(
    @Param1 nvarchar(10), 
    @Param2 int = NULL
)
RETURNS TABLE
AS 
RETURN 
   SELECT @Param1 AS Col1, @Param2 AS Col2;
GO

Then you can use it the following way (option 1):

SELECT * FROM dbo.UFN_SAMPLE_FUNCTION ('ABC', DEFAULT);

which is correct way and you get the following result:

Col1       Col2
---------- -----------
ABC        NULL

But if you try to use parametrized query (option 2):

exec sp_executesql N'SELECT * FROM dbo.UFN_SAMPLE_FUNCTION (@P1, @P2)',N'@P1 nvarchar(10),@P2 int',@P1=N'abc',@P2=default;

you will get an error:

Msg 8178, Level 16, State 1, Line 0
The parameterized query '(@P1 nvarchar(10),@P2 int)SELECT * FROM dbo.UFN_SAMPLE_FUNCTION' expects the parameter '@P2', which was not supplied.

If you have the following .net code:

public void RunTVF(string param1, int? param2)
{
    using (SqlConnection con = GetProdConection())
    {
        using (var cmd = new SqlCommand("SELECT * FROM dbo.UFN_SAMPLE_FUNCTION (@P1, @P2)", con))
        {
            cmd.CommandType = CommandType.Text;
            var param = new SqlParameter
            {
                ParameterName = "@P1",
                SqlDbType = SqlDbType.NVarChar,
                Size = 10   ,
                Value = param1
            };
            cmd.Parameters.Add(param);
            param = new SqlParameter
            {
                ParameterName = "@P2",
                SqlDbType = SqlDbType.Int,
                Value = param2
            };
            cmd.Parameters.Add(param);

            cmd.Connection.Open();
            using (IDataReader dataReader = cmd.ExecuteReader())
            {
                //...
            }
        }
    }
}

then, in case param2 = null as Jack suggested above, the script produced by the code will be identical to the option 2 and will result to the same error. So you cannot use NULL in this case.You cannot set DEFAULT as the value of SQLParameter either.

What you can do is to create a stored procedure to wrap the call to your funcion and move your default value from the function to the SP. Example:

CREATE PROCEDURE dbo.USP_SAMPLE_PROCEDURE
( 
    @Param1 nvarchar(10), 
    @Param2 int = NULL, --DEFAULT value now is here (remove it from the function)
    @Statement nvarchar(max)
)
AS
BEGIN
    SET NOCOUNT ON;
    EXEC sp_executesql @Statement,N'@P1 nvarchar(10),@P2 int',@P1=@Param1,@P2=@Param2;
END

The .NET code will look the following way:

public void RunWrapper(string param1, int? param2)
{
    using (SqlConnection con = GetProdConection())
    {
        using (var cmd = new SqlCommand("USP_SAMPLE_PROCEDURE", con))
        {
            cmd.CommandType = CommandType.StoredProcedure;
            var param = new SqlParameter
            {
                ParameterName = "@Param1",
                SqlDbType = SqlDbType.NVarChar,
                Size = 10,
                Value = param1
            };
            cmd.Parameters.Add(param);
            param = new SqlParameter
            {
                ParameterName = "@Param2",
                SqlDbType = SqlDbType.Int,
                Value = param2
            };
            cmd.Parameters.Add(param);
            param = new SqlParameter
            {
                ParameterName = "@Statement",
                SqlDbType = SqlDbType.NVarChar,
                Size = -1, //-1 used in case you need to specify nvarchar(MAX)
                Value = "SELECT * FROM dbo.UFN_SAMPLE_FUNCTION (@P1, @P2)"
            };
            cmd.Parameters.Add(param);

            cmd.Connection.Open();
            using (IDataReader dataReader = cmd.ExecuteReader())
            {
                //...
            }
        }
    }
}

In this case null as a value for the param2 will be translated to the correct DEFAULT and the following script will be produced:

exec USP_SAMPLE_PROCEDURE @Param1=N'ABC',@Param2=default,@Statement=N'SELECT * FROM dbo.UFN_SAMPLE_FUNCTION (@P1, @P2)'

which will give you the following result:

Col1       Col2
---------- -----------
ABC        NULL

I am not sure that this is the best practice. This is just the work-around.

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Though you can't set an SQL keyword as the value of a parameter, you could in this case go and get the DEFAULT VALUE.

 SELECT COLUMN_DEFAULT FROM INFORMATION_SCHEMA.COLUMNS 
      WHERE TABLE_NAME = 'table_name' AND COLUMN_NAME = 'column_name'"

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if you pass a dot net null value as the parameter value it will use sql DEFAULT if you pass a dot net DBNull.Value it will use sql NULL