Setting a member without the "null reference" spam

Class1.cs

using System;
using System.Linq.Expressions;
using System.Reflection;
using Xunit;
namespace ReflectedConstructor {
    public static class MemberExpressions {
        public static MemberInfo GetMemberInfo(this Expression expression) {
            var lambda = (LambdaExpression)expression;

            MemberExpression memberExpression;
            if (lambda.Body is UnaryExpression) {
                var unaryExpression = (UnaryExpression)lambda.Body;
                memberExpression = (MemberExpression)unaryExpression.Operand;
            } else
                memberExpression = (MemberExpression)lambda.Body;

            return memberExpression.Member;
        }
    }
    public static class ClassUtils {
        public static void SetMember<TClass, TMember>(this TClass theClass, Expression<Func<TMember>> member, TMember value) where TClass : class {
            var memberName = member.GetMemberInfo().Name;

            if (value == null) {
                throw new ArgumentNullException(memberName.Replace('_', ' ').Trim());
            }

            var field = theClass.GetType().GetField(memberName, BindingFlags.NonPublic | BindingFlags.Instance);

            if (field == null) {
                throw new InvalidOperationException("Field does not seem to exist.");
            }
            field.SetValue(theClass, value);
        }
    }

    interface IAmNotImplemented { }
    interface IFirstImplementation { }
    class AmNotImplemented : IAmNotImplemented { }
    class FirstImplementation : IFirstImplementation { }

    class UsingImplementation {
        readonly IAmNotImplemented _notImplemented;
        readonly IFirstImplementation _firstImplementation;
        public UsingImplementation(IAmNotImplemented notImplemented, IFirstImplementation firstImplementation) {
            this.SetMember(() => _notImplemented, notImplemented);
            this.SetMember(() => _firstImplementation, firstImplementation);
        }
    }


    public class TestClass {
        [Fact]
        public void TheTest() {
            var something = new UsingImplementation(new AmNotImplemented(), new FirstImplementation());
        }
    }
}