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POExtending the C# Coalesce Operator
 

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    <p>Before I explain what I want to do, if you look at the following code, would you understand what it's supposed to do? <em>(updated - see below)</em></p> <pre><code>Console.WriteLine( Coalesce.UntilNull(getSomeFoo(), f =&gt; f.Value) ?? "default value"); </code></pre> <p>C# already has a null-coalescing operator that works quite well on simple objects but doesn't help if you need to access a member of that object.</p> <p>E.g.</p> <pre><code>Console.WriteLine(getSomeString()??"default"); </code></pre> <p>works very well but it won't help you here:</p> <pre><code>public class Foo { public Foo(string value) { Value=value; } public string Value { get; private set; } } // this will obviously fail if null was returned Console.WriteLine(getSomeFoo().Value??"default"); // this was the intention Foo foo=getSomeFoo(); Console.WriteLine(foo!=null?foo.Value:"default"); </code></pre> <p>Since this is something that I come across quite often I thought about using an extension method <em>(old version)</em>:</p> <pre><code>public static class Extension { public static TResult Coalesce&lt;T, TResult&gt;(this T obj, Func&lt;T, TResult&gt; func, TResult defaultValue) { if (obj!=null) return func(obj); else return defaultValue; } public static TResult Coalesce&lt;T, TResult&gt;(this T obj, Func&lt;T, TResult&gt; func, Func&lt;TResult&gt; defaultFunc) { if (obj!=null) return func(obj); else return defaultFunc(); } } </code></pre> <p>Which allows me to write:</p> <pre><code>Console.WriteLine(getSomeFoo().Coalesce(f =&gt; f.Value, "default value")); </code></pre> <p>So would you consider this code to be readable? Is Coalesce a good name?</p> <p>Edit 1: removed the brackets as suggested by Marc</p> <h2>Update</h2> <p>I really liked lassevk's suggestions and Groo's feedback. So I added overloads and didn't implement it as an extension method. I also decided that defaultValue was redundant because you could just use the existing ?? operator for that.</p> <p>This is the revised class:</p> <pre><code>public static class Coalesce { public static TResult UntilNull&lt;T, TResult&gt;(T obj, Func&lt;T, TResult&gt; func) where TResult : class { if (obj!=null) return func(obj); else return null; } public static TResult UntilNull&lt;T1, T2, TResult&gt;(T1 obj, Func&lt;T1, T2&gt; func1, Func&lt;T2, TResult&gt; func2) where TResult : class { if (obj!=null) return UntilNull(func1(obj), func2); else return null; } public static TResult UntilNull&lt;T1, T2, T3, TResult&gt;(T1 obj, Func&lt;T1, T2&gt; func1, Func&lt;T2, T3&gt; func2, Func&lt;T3, TResult&gt; func3) where TResult : class { if (obj!=null) return UntilNull(func1(obj), func2, func3); else return null; } public static TResult UntilNull&lt;T1, T2, T3, T4, TResult&gt;(T1 obj, Func&lt;T1, T2&gt; func1, Func&lt;T2, T3&gt; func2, Func&lt;T3, T4&gt; func3, Func&lt;T4, TResult&gt; func4) where TResult : class { if (obj!=null) return UntilNull(func1(obj), func2, func3, func4); else return null; } } </code></pre> <p>Sample usage:</p> <pre><code>Console.WriteLine( Coalesce.UntilNull(getSomeFoo(), f =&gt; f.Value) ?? "default value"); </code></pre> <p>Another sample:</p> <pre><code>public class Bar { public Bar Child { get; set; } public Foo Foo { get; set; } } Bar bar=new Bar { Child=new Bar { Foo=new Foo("value") } }; // prints "value": Console.WriteLine( Coalesce.UntilNull(bar, b =&gt; b.Child, b =&gt; b.Foo, f =&gt; f.Value) ?? "null"); // prints "null": Console.WriteLine( Coalesce.UntilNull(bar, b =&gt; b.Foo, f =&gt; f.Value) ?? "null"); </code></pre>
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    1. COWow! I knew that ext. methods are basically "thiscall" static methods, but I still thought that they would throw a Null-reference exception when invoked on a null instance (because non-virtual instance methods actually work the same way)... I guess it has more sense this way anyway.
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    2. COThanks for your comment! This is exactly what I was looking for - or rather not looking for ;) An API should follow the "The Principle of Least Surprise" - so I guess using extension methods is not a good idea - I'll rethink my design!
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