c# - 在 C# 中，Type.FullName 何时返回 null？

Question

MSDN for Type.FullName说这个属性返回

null if the current instance represents a generic type parameter, an array type, pointer type, or byreftype based on a type parameter, or a generic type that is not a generic type definition but contains unresolved type parameters.

我数了五种情况，发现一个比一个更不清楚。这是我尝试构建每个案例的示例。

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication {
  public static class Program {

    public static void Main(string[] args) {
      GenericTypeParameter();
      ArrayType();
      PointerType();
      ByRefTypeBasedOnTypeParameter();
      NongenericTypeDefinitionWithUnresolvedTypeParameters();
      Console.ReadKey();
    }

    public static void GenericTypeParameter() {
      var type = typeof(IEnumerable<>)
        .GetGenericArguments()
        .First();
      PrintFullName("Generic type parameter", type);
    }

    public static void ArrayType() {
      var type = typeof(object[]);
      PrintFullName("Array type", type);
    }

    public static void PointerType() {
      var type = typeof(int*);
      PrintFullName("Pointer type", type);
    }

    public static void ByRefTypeBasedOnTypeParameter() {
      var type = null;
      PrintFullName("ByRef type based on type parameter", type);
    }

    private static void NongenericTypeDefinitionWithUnresolvedTypeParameters() {
      var type = null;
      PrintFullName("Nongeneric type definition with unresolved type parameters", type);
    }

    public static void PrintFullName(string name, Type type) {
      Console.WriteLine(name + ":");
      Console.WriteLine("--Name: " + type.Name);
      Console.WriteLine("--FullName: " + (type.FullName ?? "null"));
      Console.WriteLine();
    }
  }
}

有这个输出。

Generic type parameter:
--Name: T
--FullName: null

Array type:
--Name: Object[]
--FullName: System.Object[]

Pointer type:
--Name: Int32*
--FullName: System.Int32*

ByRef type based on type parameter:
--Name: Program
--FullName: ConsoleApplication.Program

Nongeneric type definition with unresolved type parameters:
--Name: Program
--FullName: ConsoleApplication.Program

我只有五分之一，有两个“空白”。

问题

Can someone modify my code to give simple examples of each way in which Type.FullName can be null?

Answer 1

所以我立即注意到 MSDN 引用在其案例列表中两次包含“或”，但我花了很长时间才意识到原因。现实情况是，存在三个主要案例，其中三个案例被拆分为另外三个案例。使用更清晰的标点符号，案例是

1. 一个泛型类型参数；
2. 数组类型、指针类型或基于类型参数的byref类型；或
3. 不是通用类型定义但包含未解析类型参数的通用类型。

我理解第一种情况，Rahul 的回答将我定向到 this MSDN blog post这解释了最后一个案例并给出了两个例子，现在我可以给出其余案例的例子。

using System;
using System.Linq;

namespace ConsoleApplication {

  public class GenericClass<T> {
    public void ArrayMethod(T[] parameter) { }
    public void ReferenceMethod(ref T parameter) { }
  }

  public class AnotherGenericClass<T> : GenericClass<T> { }

  public static class Program {

    public static void Main(string[] args) {
      GenericTypeParameter();
      ArrayTypeBasedOnTypeParameter();
      PointerTypeBasedOnTypeParameter();
      ByRefTypeBasedOnTypeParameter();
      NongenericTypeDefinitionWithUnresolvedTypeParameters();
      Console.ReadKey();
    }

    public static void GenericTypeParameter() {
      var type = typeof(GenericClass<>)
        .GetGenericArguments()
        .First();
      PrintFullName("Generic type parameter", type);
    }

    public static void ArrayTypeBasedOnTypeParameter() {
      var type = typeof(GenericClass<>)
        .GetMethod("ArrayMethod")
        .GetParameters()
        .First()
        .ParameterType;
      PrintFullName("Array type based on type parameter", type);
    }

    /*
     * Would like an actual example of a pointer to a generic type,
     * but this works for now.
     */
    public static void PointerTypeBasedOnTypeParameter() {
      var type = typeof(GenericClass<>)
        .GetGenericArguments()
        .First()
        .MakePointerType();
      PrintFullName("Pointer type based on type parameter", type);
    }

    public static void ByRefTypeBasedOnTypeParameter() {
      var type = typeof(GenericClass<>)
        .GetMethod("ReferenceMethod")
        .GetParameters()
        .First()
        .ParameterType;
      PrintFullName("ByRef type based on type parameter", type);
    }

    private static void NongenericTypeDefinitionWithUnresolvedTypeParameters() {
      var type = typeof(AnotherGenericClass<>).BaseType;
      PrintFullName("Nongeneric type definition with unresolved type parameters", type);
    }

    public static void PrintFullName(string name, Type type) {
      Console.WriteLine(name + ":");
      Console.WriteLine("--Name: " + type.Name);
      Console.WriteLine("--FullName: " + (type.FullName ?? "null"));
      Console.WriteLine();
    }
  }
}

/***Output***
Generic type parameter:
--Name: T
--FullName: null

Array type based on type parameter:
--Name: T[]
--FullName: null

Pointer type based on type parameter:
--Name: T*
--FullName: null

Byref type based on type parameter:
--Name: T&
--FullName: null

Nongeneric type definition with unresolved type parameters:
--Name: GenericClass`1
--FullName: null
***Output***/