Well, I have found the solution. In short:

[SqlFunction(IsDeterministic = true, Name = "ufn_RunningTotalForDecimal")]

[return: Microsoft.SqlServer.Server.SqlFacet(Precision = 18, Scale = 2)]

public static SqlDecimal RunningTotalsDecimal(

[Microsoft.SqlServer.Server.SqlFacet(Precision = 18, Scale = 2)]

SqlDecimal val,

SqlByte id,

[Microsoft.SqlServer.Server.SqlFacet(Precision = 18, Scale = 2)]

SqlDecimal nullValue

)

{

string dataName = string.Format("MulstiSqlRt_{0}_{1}", typeof(SqlByte).FullName, id.IsNull ? 0 : id.Value);

object lastSum = CallContext.GetData(dataName);

SqlDecimal total = lastSum != null ? (SqlDecimal)lastSum : SqlDecimal.Null;

if (!val.IsNull)

total = total.IsNull ? val : total + val;

else

total = total.IsNull ? nullValue : (nullValue.IsNull ? total : total + nullValue);

CallContext.SetData(dataName, total);

return total;

}

}

That required me to purchase membership on http://www.experts-exchange.com....

Best,

Darek