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    <p>This practice is somewhat questionable, so you might want to rethink your approach here.</p> <p>If you still want to go ahead with this, the correct way is to have a <code>virtual</code> method that provides this value to the base constructor:</p> <pre><code>class Base { public Base() { obj = new ObjNeedParam(GetParamVal()); } protected virtual int GetParamVal() { return 1; } } class Derived : Base { protected override int GetParamVal() { return 2; } } </code></pre> <p><strong>You will have to ensure that the <code>override</code> does not use any class members of <code>Derived</code> that get initialized in <code>Derived</code>'s constructor</strong> because that constructor will not have run yet at the point where the override is called! See <a href="https://stackoverflow.com/questions/119506/virtual-member-call-in-a-constructor">Virtual member call in a constructor</a>.</p> <p><strong>Update:</strong> It is somewhat questionable because when constructing an object of some derived type the compiler has a problem to solve: in what order should the fields be initialized and the constructors run so that any method call from inside any constructor is guaranteed to find the object in a "usable" state?</p> <p>This problem cannot be solved in the general case because by definition the object is not in a usable state until all constructors have returned, and we are talking about calling methods from <em>inside</em> these constructors. So the compiler has to guarantee as much as it can and either prevent or allow you to do things that are not provably safe.</p> <p>In this scenario C++ (amusingly) prevents the doing of things that are not provably safe, while C# allows it. Therefore it's somewhat questionable because if you don't pay attention it is possible to introduce bugs. A safer approach would be to write the class such that this member is initialized after the constructors have run.</p>
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    1. COgreat, this works! But can you please elaborate on the "somewhat questionable" please? If it's not the right way to go, I would love to know why. Thx!
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    2. CO@zorbar: Updated the answer, take a look.
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    3. COI get it. Thx a lot for the detailed explanantion.
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