Read http://msdn.microsoft.com/en-us/library/ms172138.aspx

I see that you need to explicitly assign corresponding NULL value to a parameter using the proper type.

>WinForms
>
>
>>>I have the following piece of code:
>>>
>>>

>>>SqlParameter PartDesc = new SqlParameter();
>>>PartDesc.SqlDbType = SqlDbType.VarChar;
>>>PartDesc.ParameterName = "@Part_Description";
>>>PartDesc.Value = txtPartDesc.Text;
>>>

>>>
>>>If the user didn't type anything into the txtPartDesc field, how would ensure that NULL was passed to the stored procedure?
>>
>>Are you using ASP.NET or Windows Forms? I know that for ASP.NET you can specify special property on SQLDataSource parameter to convert empty string to NULL.