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Hello.
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The next interview question we're going to discuss is "What is the difference between "is" and "as" keywords?"
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The "is" keyword is used to check if an object is of a given type.
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It returns a Boolean as a result.
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In this case, the result will be true because the text variable is a string. We can use the "is" keyword
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with value types and reference types as well.
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In this case, the result will be false because the text variable is not an integer. The "is" keyword
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is most often used to ensure that a type can be safely cast to some other type.
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We can also have some business logic driven by the type of some variable.
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In the case of type A, we want to execute different logic
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then in the case of type B. The "as" keyword is used to cast a variable to a given type.
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Here I am casting an object to a string.
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The cast will be successful and the result will be the string "Hello!".
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If the cast would not be successful, the result would be null.
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As you can see, the first variable has a value.
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But the second is null.
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The fact that in the case of invalid casting, the result will be null is the crucial difference between
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casting with "as" keyword and regular casting with parentheses.
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Regular casting throws an exception when the cast fails.
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Let's see this in code.
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This cast simply returned a null, but this caused InvalidCastException. Because casting with "as" can
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return null
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it can only be used with nullable types, so all reference types plus nullable value types.
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It's not valid with the non-nullable value types.
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This doesn't work because when casting fails (and it will in this case), we will try to assign null to
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an integer which is not legal in C# because integer is a non-nullable value type.
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Let's summarize. The "is" keyword checks
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if the object is of a given type. It returns a boolean result. The "as" keyword casts an object
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to a given type.
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It can only be used for casting to reference types or nullable value types.
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During the interview, you might be asked "What is the difference between regular cast and casting with
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"as" keyword?"
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Well, if casting with "as" fails, the result will simply be null.
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When
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regular casting fails an InvalidCastException will be thrown.
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Another question you can hear is "Why can we only use the "as" keyword to cast objects to nullable types?"
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Because if casting with "as" fails, null will be returned. Null can only be assigned to nullable types.
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All right, that's it about the "is" and "as" keywords.
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Thanks for watching, and I'll see you in the next lecture.