I did some research on this by using different methods to assign values to a nullable int. Here is what happened when I did various things. Should clarify what's going on. Keep in mind: Nullable<something> or the shorthand something? is a struct for which the compiler seems to be doing a lot of work to let us use with null as if it were a class. As you'll see below, SomeNullable == null and SomeNullable.HasValue will always return an expected true or false. Although not demonstrated below, SomeNullable == 3 is valid too (assuming SomeNullable is an int?). While SomeNullable.Value gets us a runtime error if we assigned null to SomeNullable. This is in fact the only case where nullables could cause us a problem, thanks to a combination of overloaded operators, overloaded object.Equals(obj) method, and compiler optimization and monkey business.

下面是我运行的一些代码的描述，以及它在标签中产生的输出:

int? val = null;
lbl_Val.Text = val.ToString(); //Produced an empty string.
lbl_ValVal.Text = val.Value.ToString(); //Produced a runtime error. ("Nullable object must have a value.")
lbl_ValEqNull.Text = (val == null).ToString(); //Produced "True" (without the quotes)
lbl_ValNEqNull.Text = (val != null).ToString(); //Produced "False"
lbl_ValHasVal.Text = val.HasValue.ToString(); //Produced "False"
lbl_NValHasVal.Text = (!(val.HasValue)).ToString(); //Produced "True"
lbl_ValValEqNull.Text = (val.Value == null).ToString(); //Produced a runtime error. ("Nullable object must have a value.")
lbl_ValValNEqNull.Text = (val.Value != null).ToString(); //Produced a runtime error. ("Nullable object must have a value.")

好的，让我们尝试下一个初始化方法:

int? val = new int?();
lbl_Val.Text = val.ToString(); //Produced an empty string.
lbl_ValVal.Text = val.Value.ToString(); //Produced a runtime error. ("Nullable object must have a value.")
lbl_ValEqNull.Text = (val == null).ToString(); //Produced "True" (without the quotes)
lbl_ValNEqNull.Text = (val != null).ToString(); //Produced "False"
lbl_ValHasVal.Text = val.HasValue.ToString(); //Produced "False"
lbl_NValHasVal.Text = (!(val.HasValue)).ToString(); //Produced "True"
lbl_ValValEqNull.Text = (val.Value == null).ToString(); //Produced a runtime error. ("Nullable object must have a value.")
lbl_ValValNEqNull.Text = (val.Value != null).ToString(); //Produced a runtime error. ("Nullable object must have a value.")

和以前一样。记住，初始化int?val = new int?(null);，将null传递给构造函数，将产生COMPILE时错误，因为可空对象的VALUE是不可空的。只有包装器对象本身可以等于null。

同样地，我们会从下面得到一个编译时错误:

int? val = new int?();
val.Value = null;

更不用说val.Value是一个只读属性，这意味着我们甚至不能使用这样的属性:

val.Value = 3;

但同样，多态重载隐式转换操作符让我们做到:

val = 3;

没有必要担心多物什么的，只要它工作对吗?：）

我更喜欢(a != null)，以便语法匹配引用类型。

第二种方法会有效很多倍(主要是因为编译器内联和装箱，但数字仍然非常有表现力):

public static bool CheckObjectImpl(object o)
{
    return o != null;
}

public static bool CheckNullableImpl<T>(T? o) where T: struct
{
    return o.HasValue;
}

基准测试:

BenchmarkDotNet=v0.10.5, OS=Windows 10.0.14393
Processor=Intel Core i5-2500K CPU 3.30GHz (Sandy Bridge), ProcessorCount=4
Frequency=3233539 Hz, Resolution=309.2587 ns, Timer=TSC
  [Host] : Clr 4.0.30319.42000, 64bit RyuJIT-v4.6.1648.0
  Clr    : Clr 4.0.30319.42000, 64bit RyuJIT-v4.6.1648.0
  Core   : .NET Core 4.6.25009.03, 64bit RyuJIT


        Method |  Job | Runtime |       Mean |     Error |    StdDev |        Min |        Max |     Median | Rank |  Gen 0 | Allocated |
-------------- |----- |-------- |-----------:|----------:|----------:|-----------:|-----------:|-----------:|-----:|-------:|----------:|
   CheckObject |  Clr |     Clr | 80.6416 ns | 1.1983 ns | 1.0622 ns | 79.5528 ns | 83.0417 ns | 80.1797 ns |    3 | 0.0060 |      24 B |
 CheckNullable |  Clr |     Clr |  0.0029 ns | 0.0088 ns | 0.0082 ns |  0.0000 ns |  0.0315 ns |  0.0000 ns |    1 |      - |       0 B |
   CheckObject | Core |    Core | 77.2614 ns | 0.5703 ns | 0.4763 ns | 76.4205 ns | 77.9400 ns | 77.3586 ns |    2 | 0.0060 |      24 B |
 CheckNullable | Core |    Core |  0.0007 ns | 0.0021 ns | 0.0016 ns |  0.0000 ns |  0.0054 ns |  0.0000 ns |    1 |      - |       0 B |

基准测试代码:

public class BenchmarkNullableCheck
{
    static int? x = (new Random()).Next();

    public static bool CheckObjectImpl(object o)
    {
        return o != null;
    }

    public static bool CheckNullableImpl<T>(T? o) where T: struct
    {
        return o.HasValue;
    }

    [Benchmark]
    public bool CheckObject()
    {
        return CheckObjectImpl(x);
    }

    [Benchmark]
    public bool CheckNullable()
    {
        return CheckNullableImpl(x);
    }
}

使用https://github.com/dotnet/BenchmarkDotNet

因此，如果你有一个选项(例如编写自定义序列化器)来处理Nullable在不同的管道与对象-并使用他们的特定属性-这样做，并使用Nullable特定的属性。 因此，从一致的思维角度来看，HasValue应该是首选。一致的思考可以帮助你写出更好的代码，不要在细节上花费太多时间。

附言:人们说“因为思维一致，所以更喜欢HasValue”的建议是毫无意义的。你能预测它的性能吗?

public static bool CheckNullableGenericImpl<T>(T? t) where T: struct
{
    return t != null; // or t.HasValue?
}

PPS人们继续减，似乎没有人试图预测CheckNullableGenericImpl的性能。我会告诉你:编译器不会帮助你用HasValue替换!=null。如果对性能感兴趣，应该直接使用HasValue。

I did some research on this by using different methods to assign values to a nullable int. Here is what happened when I did various things. Should clarify what's going on. Keep in mind: Nullable<something> or the shorthand something? is a struct for which the compiler seems to be doing a lot of work to let us use with null as if it were a class. As you'll see below, SomeNullable == null and SomeNullable.HasValue will always return an expected true or false. Although not demonstrated below, SomeNullable == 3 is valid too (assuming SomeNullable is an int?). While SomeNullable.Value gets us a runtime error if we assigned null to SomeNullable. This is in fact the only case where nullables could cause us a problem, thanks to a combination of overloaded operators, overloaded object.Equals(obj) method, and compiler optimization and monkey business.

下面是我运行的一些代码的描述，以及它在标签中产生的输出:

int? val = null;
lbl_Val.Text = val.ToString(); //Produced an empty string.
lbl_ValVal.Text = val.Value.ToString(); //Produced a runtime error. ("Nullable object must have a value.")
lbl_ValEqNull.Text = (val == null).ToString(); //Produced "True" (without the quotes)
lbl_ValNEqNull.Text = (val != null).ToString(); //Produced "False"
lbl_ValHasVal.Text = val.HasValue.ToString(); //Produced "False"
lbl_NValHasVal.Text = (!(val.HasValue)).ToString(); //Produced "True"
lbl_ValValEqNull.Text = (val.Value == null).ToString(); //Produced a runtime error. ("Nullable object must have a value.")
lbl_ValValNEqNull.Text = (val.Value != null).ToString(); //Produced a runtime error. ("Nullable object must have a value.")

好的，让我们尝试下一个初始化方法:

int? val = new int?();
lbl_Val.Text = val.ToString(); //Produced an empty string.
lbl_ValVal.Text = val.Value.ToString(); //Produced a runtime error. ("Nullable object must have a value.")
lbl_ValEqNull.Text = (val == null).ToString(); //Produced "True" (without the quotes)
lbl_ValNEqNull.Text = (val != null).ToString(); //Produced "False"
lbl_ValHasVal.Text = val.HasValue.ToString(); //Produced "False"
lbl_NValHasVal.Text = (!(val.HasValue)).ToString(); //Produced "True"
lbl_ValValEqNull.Text = (val.Value == null).ToString(); //Produced a runtime error. ("Nullable object must have a value.")
lbl_ValValNEqNull.Text = (val.Value != null).ToString(); //Produced a runtime error. ("Nullable object must have a value.")

和以前一样。记住，初始化int?val = new int?(null);，将null传递给构造函数，将产生COMPILE时错误，因为可空对象的VALUE是不可空的。只有包装器对象本身可以等于null。

同样地，我们会从下面得到一个编译时错误:

int? val = new int?();
val.Value = null;

更不用说val.Value是一个只读属性，这意味着我们甚至不能使用这样的属性:

val.Value = 3;

但同样，多态重载隐式转换操作符让我们做到:

val = 3;

没有必要担心多物什么的，只要它工作对吗?：）

在VB。Net，当你可以使用. hasvalue时，不要使用IsNot Nothing。我刚刚解决了一个“操作可能会破坏运行时”的中等信任错误，将IsNot Nothing替换为. hasvalue。我不太明白为什么，但是在编译器中发生了一些不同的事情。我认为c#中的!= null可能有同样的问题。

编译器将null比较替换为对HasValue的调用，因此没有真正的区别。选择对你和你的同事来说更有可读性/更有意义的就行。