Understanding When Type.FullName Returns Null in .NET

A colleague recently asked me an interesting question: "Why does Type.FullName return null in certain situations?" Indeed, the method signature is

public abstract string? FullName { get; }

This behavior might seem unexpected, but there are specific scenarios where the .NET runtime cannot generate a valid full name for a type. Here are the two main cases where Type.FullName returns null:

#Generic Types with Open Type Parameters

When you create a generic type that contains unbound generic parameters:

var list = typeof(IList<>);
var dict = typeof(IDictionary<,>);
var listOfDictionaries = list.MakeGenericType(dict); // IList<IDictionary<,>>
Assert.Null(listOfDictionaries.FullName);

#Function Pointers

Function pointer types introduced in C# 9 also have null as their FullName:

var functionPointerType = typeof(delegate*<int, void>);
Assert.Null(functionPointerType.FullName);

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