I think you want the CodeTypeReference
something like this: (Note that I haven't tested this fully, I've just created the graph - not actually rendered it to code)
cas.Right = new CodeMethodInvokeExpression(
new CodeMethodReferenceExpression(
new CodeVariableReferenceExpression("row"),
"Field",
new CodeTypeReference("System.String")),
new CodePrimitiveExpression(0));