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POComparing structs to null
 

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    <blockquote> <p><strong>Possible Duplicate:</strong><br> <a href="https://stackoverflow.com/questions/1972262/c-okay-with-comparing-value-types-to-null">C# okay with comparing value types to null</a> </p> </blockquote> <p>I was working on a windows app in a multithreaded environment and would sometimes get the exception "Invoke or BeginInvoke cannot be called on a control until the window handle has been created." So I figured that I'd just add this line of code:</p> <pre><code>if(this.Handle != null) { //BeginInvokeCode } </code></pre> <p>But that didn't solve the problem. So I dug a little further, and realized that IntPtr (the type that Form.Handle is) is a struct which can't be nullable. This was the fix that worked:</p> <pre><code>if(this.Handle != IntPtr.Zero) { //BeginInvokeCode } </code></pre> <p>So then it hit me, why did it even compile when I was checking it for null? So I decided to try it myself:</p> <pre><code> public struct Foo { } </code></pre> <p>and then:</p> <pre><code> static void Main(string[] args) { Foo f = new Foo(); if (f == null) { } } </code></pre> <p>and sure enough it didn't compile saying that "Error 1 Operator '==' cannot be applied to operands of type 'ConsoleApplication1.Foo' and ''". Ok, so then I started looking at the metadata for IntPtr and started adding everything to my Foo struct that was there in the IntPtr struct (ISerializable, ComVisible) but nothing helped. Finally, when I added the operator overloading of == and !=, it worked:</p> <pre><code>[Serializable] [ComVisible(true)] public struct Foo : ISerializable { #region ISerializable Members public void GetObjectData(SerializationInfo info, StreamingContext context) { throw new NotImplementedException(); } #endregion public override bool Equals(object obj) { return base.Equals(obj); } public override int GetHashCode() { return base.GetHashCode(); } public static bool operator ==(Foo f1, Foo f2) { return false; } public static bool operator !=(Foo f1, Foo f2) { return false; } } </code></pre> <p>This finally compiled:</p> <pre><code> static void Main(string[] args) { Foo f = new Foo(); if (f == null) { } } </code></pre> <p>My question is why? Why if you override == and != are you allowed to compare to null? The parameters to == and != are still of type Foo which aren't nullable, so why's this allowed all of a sudden?</p>
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    1. CO+1 for good question btw, and hello from fellow Brooklynite :)
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