What is the purpose of isnullorempty in C#?

The `IsNullOrEmpty` method in C# is used to check whether a specified string is null or empty. It is used to determine if a string variable is empty or has a length of zero. `IsNullOrEmpty` is a static method of the `string` class, which can be directly called using the class name, for example `string.IsNullOrEmpty(str)`, where `str` is the string variable to be checked. The method returns a value of type `bool`, returning `true` if the given string is null or empty, and `false` otherwise. Using `IsNullOrEmpty` method can help avoid multiple conditional checks when determining if a string is empty, simplifying code writing. For example, you can check if a string is empty by using the following:

string str = null;
if (string.IsNullOrEmpty(str))
{
    Console.WriteLine("字符串为空");
}
else
{
    Console.WriteLine("字符串不为空");
}

In the code above, if the string `str` is empty, it will output “String is empty”; otherwise, it will output “String is not empty”.