如何使用表达式检查！= null对用户定义的数据类型？

Question

I'm new to expressions. I'm trying this and it doesn't seem to work.

ParameterExpression pe = Expression.Parameter(typeof(Customer));
Expression left = Expression.Property(pe, "OrderList");
Expression right = Expression.Constant(null, typeof(Nullable));
Expression res = Expression.NotEqual(left, right);

I'm gettign an "InvalidOperationException". In simple If-Else statement is look like this

if(custObj.OrderList != null)
{...}

Any help would be great.

Answer 1

Simply use

Expression right = Expression.Constant(null, left.Type);

It will give you the null constant with the same type as the left operand (property in your case).

Answer 2

The problem is you are comparing objects of different types. You can solve this by using Expression.Constant(object value) overload, I'm assuming that the type of the property OrderList is by reference, if not you can't do this comparison.

ParameterExpression pe = Expression.Parameter(typeof(Customer));
Expression left = Expression.Property(pe, "OrderList");
Expression right = Expression.Constant(null);
Expression res = Expression.NotEqual(left, right);