Question

How do I check if a given object is nullable in other words how to implement the following method…

bool IsNullableValueType(object o) {     ... } 

I am looking for nullable value types. I didn’t have reference types in mind.

//Note: This is just a sample. The code has been simplified  //to fit in a post.  public class BoolContainer {     bool? myBool = true; }  var bc = new BoolContainer();  const BindingFlags bindingFlags = BindingFlags.Public                         | BindingFlags.NonPublic                         | BindingFlags.Instance                         ;   object obj; object o = (object)bc;  foreach (var fieldInfo in o.GetType().GetFields(bindingFlags)) {     obj = (object)fieldInfo.GetValue(o); } 

obj now refers to an object of type bool (System.Boolean) with value equal to true. What I really wanted was an object of type Nullable<bool>

So now as a work around I decided to check if o is nullable and create a nullable wrapper around obj.

Answer 1

There are two types of nullable – Nullable<T> and reference-type.

Jon has corrected me that it is hard to get type if boxed, but you can with generics: – so how about below. This is actually testing type T, but using the obj parameter purely for generic type inference (to make it easy to call) – it would work almost identically without the obj param, though.

static bool IsNullable<T>(T obj) {     if (obj == null) return true; // obvious     Type type = typeof(T);     if (!type.IsValueType) return true; // ref-type     if (Nullable.GetUnderlyingType(type) != null) return true; // Nullable<T>     return false; // value-type } 

But this won’t work so well if you have already boxed the value to an object variable.

Microsoft documentation: https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/nullable-types/how-to-identify-a-nullable-type