Expressions static method requires null instance non-static method requires non-null instance

Question

I'm new to using Expressions and am getting the following error:

System.ArgumentException : Static method requires null instance, non-static method requires non-null instance.
Parameter name: method

The code is as follows:

        int inP = 100;
        object inParam = inP;
        Type inParamType = inParam.GetType();

        ParameterExpression pe = Expression.Parameter(typeof(S), "pe");

        Expression left = Expression.Property(pe, typeof(S).GetProperty(propName));
        Expression right = Expression.Constant(inParam, inParamType);

        MethodInfo mi = inParamType.GetMethod(operand, BindingFlags.Instance | BindingFlags.Public, null, new Type[] { typeof(object) }, null);
        Expression e1 = Expression.Call(mi, left, right);

user743382 · Accepted Answer · 2016-01-05 18:53:33Z

19

You're using BindingFlags.Instance, so you'll only get back instance methods. Instance methods must be called as C# a.f(b), not f(a, b), and that translates to expression tree Expression.Call(left, mi, right), not Expression.Call(mi, left, right). That's what the exception is telling you:

Static method requires null instance, non-static method requires non-null instance.

In this case you have a non-static method, therefore you must pass in an instance on which to call the method.

answered Jan 5, 2016 at 18:53

user743382

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2 Comments

Robert Harvey Over a year ago

If I read this correctly, you're suggesting to use one of the overloads of Expression.Call that takes an instance (represented as an expression) as the first parameter. In my case, I was able to do this by passing Expression.Constant(instance) to Expression.Call, as the first parameter. Does this sound correct to you?

user743382 Over a year ago

@RobertHarvey If the instance is not variable, for instance if you want to construct Expression<Func<object, object>> e = a => instance.f(a); manually, then you can in theory construct the instance part of that by passing Expression.Constant(instance) as the first argument to Expression.Call, but in practice, I've noticed that since the C# compiler doesn't do this (it turns it into a field in a compiler-generated class instead), some query providers don't support it, unfortunately.

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