这是C#的语法糖。其实这个语法在CLR编译器中被转化为：
Nullable<DateTime>。是一个值类型，可空类型。
表示这个东西可以赋值为null。
你或许感到奇怪——为何值类型Nullable<DateTime>可以赋值null？
比如：
Nullable<DateTime> dt = null;
//其实被编译器理解为：
Nullable<DateTime> dt = new Nullable<DateTime>();
//但是注意源码：
[Serializable, StructLayout(LayoutKind.Sequential), TypeDependency("System.Collections.Generic.NullableEqualityComparer`1"), TypeDependency("System.Collections.Generic.NullableComparer`1")]
public struct Nullable<T> where T: struct
{
private bool hasValue;
internal T value;
public Nullable(T value)
{
this.value = value;
this.hasValue = true;
}
public bool HasValue
{
get
{
return this.hasValue;
}
}
public T Value
{
get
{
if (!this.HasValue)
{
ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NoValue);
}
return this.value;
}
}
public T GetValueOrDefault()
{
return this.value;
}
public T GetValueOrDefault(T defaultValue)
{
if (!this.HasValue)
{
return defaultValue;
}
return this.value;
}
public override bool Equals(object other)
{
if (!this.HasValue)
{
return (other == null);
}
if (other == null)
{
return false;
}
return this.value.Equals(other);
}
public override int GetHashCode()
{
if (!this.HasValue)
{
return 0;
}
return this.value.GetHashCode();
}
public override string ToString()
{
if (!this.HasValue)
{
return "";
}
return this.value.ToString();
}
public static implicit operator T?(T value)
{
return new T?(value);
}
public static explicit operator T(T? value)
{
return value.Value;
}
}
注意：值类型默认就有空构造函数，因此你不必显示定义！
因此空构造函数其实把hasValue定义为空了。所以本质上Nullable<T>还是保存了数值的，
只是人为用HasValue屏蔽了。
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