If a parameter is NULL or DBNull then its size needs to be zero other…

…wise the parameter gets treated as an empty string which is a different thing to a NULL. Microsoft will send a true NULL, this will get mono to do the same.

mcs/class/Mono.Data.Tds/Mono.Data.Tds.Protocol/Tds70.cs

@@ -522,6 +522,13 @@ private void WriteParameterInfo (TdsMetaParameter param)
 				size = param.GetActualSize ();
 			}
 
+			/*
+			 * If the value is null, not setting the size to 0 will cause varchar
+			 * fields to get inserted as an empty string rather than an null.
+			 */
+			if (param.Value == null || param.Value == DBNull.Value)
+				size = 0;
+
 			// Change colType according to the following table
 			/* 
 			 * Original Type	Maxlen		New Type