How to create an instance of a Nullable<T>?
c#, generics, linq, reflection
Solution
You should try `Expression.New`:
var ifTrue = Expression.New(typeof(Nullable<int>));
Problem
Suppose we have two classes: ``` public class ParentEntity { public ChildEntity Child { get; set; } } public class ChildEntity { public byte? NullableValue { get; set; } public byte Value { get; set; } } ``` The task is to express the following code in linq expressions: ``` parent.Child == null ? null : parent.Child.NullableValue ``` To do that I tried to use the following code: ``` public static Expression GetCondition<TParent, TChild, TChildKey>( Expression<Func<TParent, TChild>> pe, Expression<Func<TChild, TChildKey>> ce) { var test = Expression.Equal(pe.Body, Expression.Constant(null)); var ifTrue = Expression.Constant(Activator.CreateInstance<TChildKey>()); var ifFalse = Expression.Property(pe.Body, (ce.Body as MemberExpression).Member.Name); return Expression.Condition(test, ifTrue, ifFalse); } ``` Running this code with ``` Expression<Func<ParentEntity, ChildEntity>> pe = n => n.Child; GetCondition(pe, n => n.Value); // ok GetCondition(pe, n => n.NullableValue); // throws an ArgumentException ``` throws an `ArgumentException` at the last line (at the return statement in the `GetCondition`), saying that the types of arguments are mismatched. Having analysed this code I found out that `Activator.CreateInstance<TChildKey>()` returns `object`, but not `TChildKey`, when `TChildKey` is `System.Nullable<T>`, i.e. `ifTrue.Type` is `object`, while I expected it to be `System.Nullable<byte>`. This very issue is discussed in SO: Creating a nullable object via Activator.CreateInstance returns null pointing to Reflection and Nullable But none of these suggests anything to solve the issue. Is there a way to create an instance of exactly `System.Nullable<T>` type, having value `null`? Or maybe there is some other way to express the original conditional expression?