Wednesday, August 22, 2012

VAR vs DYNAMIC in c# -- first look

If you declare AND initialize a variableA of "var" type, compiler figures out the exact type of this variableA. Say it's Account. This variableA will be this type forever. In fact, compiler must get enough info to infer the exact type of variableA. Otherwise compile time error.

If variableB is declared "dynamic" type, then you can declare it without initializing. Then at run time you can assign an Account object to it, subsequently assign an integer to it. The actual type of variableB is mutable at run time. This is a bold departure from c++/java, where every variable's type is always immutable.

You can get run time error (not compile time) with variableB, if you reference a member of variableB and the member doesn't exist at time of evaluation, which is run time. Put another way, Evaluation is NOT at compile time. See simple example at http://msdn.microsoft.com/en-us/library/dd264741.aspx. In this sense, dynamic type is more lenient than Object type as "evaluation" is postponed to the very last moment at run time.

Dynamic type was designed for COM objects.

Only local variables can be "var", but dynamic can be fields, method params etc.

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