Nullable<T> 的实现

[Serializable, StructLayout(LayoutKind.Sequential), TypeDependency("System.Collections.Generic.NullableComparer`1"), TypeDependency("System.Collections.Generic.NullableEqualityComparer`1")]
public struct Nullable<T> where T: struct
{
private bool hasValue;
internal T value;
[TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
public Nullable(T value)
{
this.value = value;
this.hasValue = true;
}

public bool HasValue
{
get
{
return this.hasValue;
}
}
public T Value
{
[TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
get
{
if (!this.HasValue)
{
ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NoValue);
}
return this.value;
}
}
[TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
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;
}
}