SQL Server -- Handling null input in CLR User-Defined Function (UDF) with OnNullCall

Question

I have a user-defined function in SQL Server (written in .NET) that cleans text. I'm wondering how to handle null input.

Here is the function in C#:

[Microsoft.SqlServer.Server.SqlFunction]
public static SqlChars cleanEstActText(SqlChars input)
{
    SqlChars cascadingSqlChar = removeNBSP(input);
    cascadingSqlChar = optimizeFontTags(cascadingSqlChar);

    return cascadingSqlChar;
}

This is the error in SQL if the function gets any null data:

A .NET Framework error occurred during execution of user-defined routine or aggregate "removeNBSP": 
System.Data.SqlTypes.SqlNullValueException: Data is Null. This method or property cannot be called on Null values.
System.Data.SqlTypes.SqlNullValueException: 
   at System.Data.SqlTypes.SqlChars.get_Value()
   at UserDefinedFunctions.removeNBSP(SqlChars input)

Reading on SO and Google led me to the OnNullCall attribute, which looks promising.

From MSDN:

true if the method is called when null (Nothing in Visual Basic) input arguments are specified in the method invocation; false if the method returns a null (Nothing in Visual Basic) value when any of its input parameters are null (Nothing in Visual Basic).

Sounds exactly like what I want; if I get null, just pass null through. I'm not quite sure how to implement it, so I check MSDN again (http://msdn.microsoft.com/en-us/library/microsoft.sqlserver.server.sqlmethodattribute.aspx), and rewrite the first line of my function from

[Microsoft.SqlServer.Server.SqlFunction]

to

[Microsoft.SqlServer.Server.SqlMethod(OnNullCall = false, IsMutator = false, InvokeIfReceiverIsNull = false)]

If I do this, I get an error in SQL any time I use it:

Cannot find either column "dbo" or the user-defined function or aggregate "dbo.cleanEstActText", or the name is ambiguous.

Am I implementing OnNullCall incorrectly? Should I be doing something else? Is there really any good way to make my function pass nulls through?

Answer 1

You can try this

[Microsoft.SqlServer.Server.SqlFunction]
public static SqlChars cleanEstActText(SqlChars input)
{

    if (input.IsNull) return null;

    SqlChars cascadingSqlChar = removeNBSP(input);
    cascadingSqlChar = optimizeFontTags(cascadingSqlChar);

    return cascadingSqlChar;
}

All Nullable SqlData Types have an IsNull Propery.

Thanks Hari

Answer 2

The accepted answer is not correct, though it does technically work. The problem with checking for NULLs in the code itself is that the code is called and has to perform that check. This is required only when wanting to allow one or more parameters to pass a valid NULL in without causing the execution of the method to be skipped.

This is definitely possible to do, though unfortunately not through the Visual Studio / SSDT publishing mechanism that creates all of the T-SQL for you. In order to accomplish this, you need to either:

• Manually deploy the T-SQL CREATE FUNCTION statement
• Do an ALTER FUNCTION after the code has been published to SQL Server

In either case, the syntax for this, as described in the MSDN page for CREATE FUNCTION, is: WITH RETURNS NULL ON NULL INPUT.

To put it in full context:

CREATE FUNCTION SchemaName.FunctionName ( { parameter_list } )
RETURNS DataType
WITH RETURNS NULL ON NULL INPUT
AS EXTERNAL NAME ...

Again, keep in mind that if this option is specified, then any input parameter that is NULL will cause the function to be skipped and return NULL.

UPDATE:
Please vote on the following Microsoft Connect suggestion so that hopefully support is added for the OnNullCall property of the SqlFunction attribute:

Implement OnNullCall property in SqlFunctionAttribute for RETURNS NULL ON NULL INPUT