Casting vs. 'as' Keyword in C#: When Should I Use Which?

C# for compulsory conversion and "AS" keyword comparison: When will it be used?

When processing the interface, developers often use mandatory conversion or object type conversion to operate objects. However, there are two different methods to perform these conversion: forced conversion and using the "AS" keywords. Is there differences between these two methods? If so, how can they affect the execution and performance of the program?

Understand the mandatory conversion

Mandatory conversion involves the target type before placing the target before the object references, in the brackets, the object is explicitly converted from one type to another. For example:

The compulsory conversion indicator compilers immediately execute the type conversion, if the conversion is invalid, it will cause abnormalities.

Explore the "AS" keyword

IMyInterface _MyObj = new MyClass();
MyClass _myCls1 = (MyClass)_MyObj;

"AS" keywords perform non -invasive types. If the conversion is unsuccessful, return NULL. However, if the conversion is successful, "AS" will return the conversion object. Compared with mandatory conversion, "AS" provides an alternative method. If the conversion fails, it will not cause abnormalities. Instead, return NULL references.

<本> Cost comparison

In the past, there was a difference in performance differences between the conversion and the use of the "AS" keywords. Forced conversion operation is faster because it only involves one -step type conversion. However, with the advancement of the modern JIT compiler, the efficiency of the "AS" keywords has been as high as the mandatory conversion. Both technologies can quickly implement the type conversion, so they can be ignored in terms of performance.

IMyInterface _MyObj = new MyClass();
MyClass _myCls2 = _MyObj as MyClass;
// 如果转换不成功，_myCls2 将为 null

Select the best method

Although performance is no longer a decisive factor, other aspects will affect the choice between mandatory conversion and "AS":

Security conversion: When processing variables, it is recommended to use "AS" because the type of variable may change between testing and conversion. This reduces abnormal risks caused by invalid conversion.

Following point separation:

"AS" keywords allow the test and conversion steps to separate. This enhances the readability of the code and reduces the possibility of minor errors.

• Modern Best Practice
In C# 7 and higher versions, the pattern matching has greatly replaced the "AS" keywords. This mechanism eliminates the demand for explicit type tests, thereby generating a more concise and declarative code.
• Conclusion

"AS" keywords and mandatory conversion provides different methods to perform type conversion in CLR. Although the mandatory conversion provides a direct and efficient type conversion, "AS" can achieve a safer conversion by returning NULL instead of abnormalities. Modern JIT compilers have minimized performance differences between these technologies, so appropriate choices can be made according to security and code structure. Generally speaking, when processing variables, and in order to separate type testing and conversion to improve code maintenance, it is recommended to use "AS".

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