Operator ?: seams to evaluate both sides of expression.

Expression xx == null ? 0 : xx.SomeProperty failed with exception “No property or field SomeProperty exists in type Object (at index 22).” The problem is that the expression xx is null. If I change the expression to “xx == null ? 1 : 2” it is evaluated and the result is 1.

[RupertAvery]

The problem is that the parser has no idea what xx is.

The parser is not evaluating xx.SomeProperty, the error is a parse-time error. You are assigning a null value to xx, and without any type information the parser cannot know what xx is. null is treated as type Object and so in the process of parsing, it tries to find SomeProperty on type Object, which of course fails.

This is why C# forbids assigning a null value using the var expression.

So you need to pass in the type information explicitly using the SetVariable(string, object, Type) overload. Suppose you have SomeClass with SomeProperty:

interpreter.SetVariable("xx", null, typeof(SomeClass));

This will make your code work as expected.

Note however that assigning null to a typed variable does not work:

SomeClass xx = null;
interpreter.SetVariable("xx", xx);

This is because a typed variable set to null will actually be passed as a null object (without type information) into SetVariable, it acts as if it had no type at all, hence the need for an overload explicitly allowing you to set the type.

[1andy11]

You are right. Using SetVariable with type parameter make it work. Thanks.