“是{}”是什么意思？

Question

I see the following code sometimes, and have no idea what the expression is actually testing.

public static void Something(string[] value)
{
   if (value is { })
   {
      DoSomethingElse();
   }
}

Answer 1

That's just the empty property pattern in C# 8, meaning the value not null . It matches any value type or reference type. As Panagiotis Kanavos notes in the comments, this is equivalent to the good old value is object check which has been in C# for a long time.

Generally if you were to specify a property, then it would match or not. This esoteric example illustrates that:

if (value is { Length: 2 })
{
   // matches any object that isn't `null` and has a property set to a length of 2
}

The property patterns work best and are most clear when comparing with other patterns in cases such as switch expressions.

Answer 2

While Daniel's answer is right, I think it might be useful to add some context about why you may see the empty property pattern in use. Consider this example controller method that needs some validation done:

public async Task<IActionResult> Update(string id, ...) 
{
    if (ValidateId(id) is { } invalid)
        return invalid;
    ...
}

In the above, ValidateId() could return null or an instance of BadObjectRequestResult . If the former is returned, the validation is successful and moves on to the rest of the body of Update . If the latter is returned, is {} is true (ie of course an instance of BadObjectRequestResult is an object ), and the validation fails.

Nicely, out of this we've also provided a variable name, invalid , which we can return immediately. Without that we'd need slightly more verbose code.

public async Task<IActionResult> Update(string id, ...) 
{
    var invalid = ValidateId(id);
    if (invalid != null)
        return invalid;
    ...
}

Whether one is more readable or the other is up to the reader, I've just presented one way the empty property pattern can be used.

Answer 3

我认为是检查值是否为空对象