Error message in XSLT with C# extension function

Question

I am received the following error while trying to implement a C# extension function in XSLT.

Extension function parameters or return values which have

Answer 1

From http://msdn.microsoft.com/en-us/library/533texsx(VS.71).aspx

The supplied arguments and return values defined by the script functions must be one of the W3C XPath or XSLT types. The following table shows the corresponding W3C types, the equivalent .NET classes (Type), and whether the W3C Type is an XPath Type or XSLT Type.