如何在 C# 中检查方法的返回类型是否可为空（启用完全可为空上下文）

Question

如何检查方法的返回类型是否可为空？ 项目设置

<Nullable>enable</Nullable>

已激活。

一些示例（所有方法均为

Task

或

Task<T>

类型）

public sealed class Dto
{
    public int Test { get; set; }
}

public sealed class Dto3
{
    public int? Test { get; set; }
}

public async Task<Dto?> GetSomething()
{
    // This should be found as "Nullable enabled" as the `?` is set.
}

public async Task<Dto3> GetSomething2()
{
    // This should not be found as "Nullable enabled" as the `?` is missing.
}

public async Task<List<Dto>> GetSomething3()
{
    // This should not be found as "Nullable enabled" as the `?` is missing
    // (And the list must be initialized anyways thanks to nullable context).
}

public async Task<List<Dto?>> GetSomething3()
{
    // This should not be found as "Nullable enabled" as the `?` is missing in the first generic type after `Task`.
}

我已经有了迭代搜索命名空间中的类和方法的代码，如下所示：

var assembly = Assembly.GetAssembly(typeof(ISomethingService));

if (assembly is null)
{
    throw new InvalidOperationException("This should never happen");
}

var classes = assembly.GetTypes().Where(type => 
 (type.Namespace == typeof(ISomethingService).Namespace || type.Namespace == typeof(ISomethingService2).Namespace) && type.IsInterface);

foreach (var @class in classes)
{
    var methods = @class.GetMethods();

    foreach (var method in methods)
    {
        foreach (var genericType in method.ReturnType.GenericTypeArguments)
        {
            if (IsNullable(genericType))
            {
                Console.WriteLine($"Return type of method {@class.Name}.{method.Name} is nullable: {genericType.Name}");
            }
        }
    }
 }

可为空检查目前是这样实现的，但没有按预期工作：

private 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
}

有没有办法达到预期的结果（例如，如果在

IsNullable

之后的第一级（泛型）设置了

true

，则只有

?

是

Task<T>

？

Answer 1

在 .NET 6+ 中，您可以使用

NullabilityInfoContext

来查明字段、属性、参数或事件是否可为 null。另请参阅此答案。

在您的情况下，您想要获取该方法的

ReturnParameter

，这是一个

PropertyInfo

。

bool ReturnNullableOrNullableTask(MethodInfo m) {
    var context = new NullabilityInfoContext();
    var returnParameter = m.ReturnParameter;
    var info = context.Create(returnParameter);
    if (info.ReadState == NullabilityState.Nullable) {
        return true; // the return type itself is null
    }
    // otherwise check if it is a Task<T>
    if (returnParameter.ParameterType == typeof(Task<>)) {
        var firstTypeParameterInfo = info.GenericTypeArguments[0];
        if (firstTypeParameterInfo.ReadState == NullabilityState.Nullable) {
            return true;
        }
    }
    return false;
}

这适用于可为空值类型和可为空引用类型。