SQLContext.WindowsIdentity.Impersonate() Fails

I am calling SqlContext.WindowsIdentity.Impersonate() within a CLR Stored Procedure and getting the following error:


System.InvalidOperationException: Data access is not allowed in this context. Either the context is a function or method not marked with DataAccessKind.Read or SystemDataAccessKind.Read, is a callback to obtain data from FillRow method of a Table Valued Function, or is a UDT validation method.

The CLR Routine does not do any kind of data accees (as far as SQL is concerned).

Here is the routine:


[SqlFunction(

DataAccess = DataAccessKind.Read,
SystemDataAccess = SystemDataAccessKind.None )] public static int CallService( string sServer, int iPort, string sPath, SqlXml cSqlXmlRequest, out SqlXml cSqlXmlResponse ) { string sXmlResponse = string.Empty; WindowsImpersonationContext cImpersonationContext = null; try {
cImpersonationContext = SqlContext.WindowsIdentity.Impersonate(); sXmlResponse = SoapTransport.CallService( sServer, iPort, sPath, cSqlXmlRequest.Value );


...
} catch (...) { throw; } finally { if ( cImpersonationContext != null ) cImpersonationContext.Undo(); } return SQL_SUCCESS; }

The only SQLContext method/properties that are called is the WindowsIdentity. Further, if I "push" the SqlContext.WindowsIdentity.Impersonate(); call inside CallService, everything work. (It also works correctly if I leave the call of and change the execution identity of SQLServer).

Any ideas as to why this error is occurring and how to resolve it?

Thanks.