Summary of some common usages of exclamation marks (!) in C#

In C#, exclamation marks (!) have multiple uses, depending on the context. Here are some common usages and examples:

• Logical non-operators: Exclamation marks can be used as logical non-operators to reverse the Boolean value. It willtrueConvert tofalse,WillfalseConvert totrue. Example:

bool isTrue = true;
bool isFalse = !isTrue; // isFalseThe value offalse

• Empty reference check: Exclamation marks can be merged with null values ​​(?.) is used together for empty reference checking. It means if the expression on the left isnull, throw itNullReferenceExceptionabnormal. Example:

string name = null;
string upperCaseName = name!.ToUpper(); // ifnamefornull，Will throwNullReferenceExceptionabnormal

• Non-null assertion operator: Exclamation marks can be used as non-null assertion operators, and when an expression is known to be notnullWhen , you can use an exclamation mark to tell the compiler not to perform empty reference checks. Example:

string message = GetMessageFromExternalSource();
string upperCaseMessage = message!.ToUpper(); // Tell the compilermessageNot fornull，No empty reference checking

• Generic constraints: The exclamation mark can be used as a constraint for a generic type parameter, indicating that the type parameter must be a non-nullable value type. Example:

public class MyClass<T> where T : struct // T must be a non-nullable value type{
    // ...
}

• Tag name: Exclamation marks can be used as part of the tag name to identify tag statements in the code. Example:

startLoop:
for (int i = 0; i < 10; i++)
{
    if (i == 5)
        goto startLoop; // Jump to the startLoop tag to continue executing the loop}

These are some common usages and examples of exclamation marks in C#. Note that the specific meaning of an exclamation mark depends on the context and may have different uses and behaviors in different situations.

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