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POMethod resolution in extension methods with optional parameters
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  1. POMethod resolution in extension methods with optional parameters
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    copied!<p>I have the following couple of extension methods, which are in the same namespace and assembly:</p> <pre><code>public static class DateTimeExtensions { public static string NullSafeToString(this DateTime? possiblyNullDateTime, string format, string nullString = "") } public static class NullableExtensions { public static string NullSafeToString&lt;T&gt;(this Nullable&lt;T&gt; nullable, string nullString = "") where T : struct } </code></pre> <p>My question is about method resolution. The following call (from another namespace) resolves to <code>ObjectExtensions.NullSafeToString</code> when I expected <code>DateTimeExtensions.NullSafeToString</code>:</p> <pre><code>DateTime? dateTime; // ... dateTime.NullSafeToString("yyyyMMdd"); </code></pre> <p>Removing the optional parameter from <code>DateTimeExtensions.NullSafeToString</code> causes it to resolve as expected.</p> <p>Section 7.6.5.2 of the C# spec outlines the order of the namespaces searched, but because the above are in the same namespace, I'd expect it to use the rules in section 7.6.5.1.</p> <p>I thought it'd match <code>DateTimeExtensions.NullSafeToString</code> because:</p> <ul> <li>Although they'll both have a first parameter type of <code>Nullable&lt;DateTime&gt;</code>, I thought a non-generic method (i.e. without the type parameter) would be considered first.</li> <li>I though the parameter lists would be considered without their optional parameters first</li> </ul> <p>Can anyone clarify why it's picking <code>ObjectExtensions.NullSafeToString</code> over <code>DateTimeExtensions.NullSafeToString</code>?</p> <p>(Aside: From other discussions on here, I suspect some people may disapprove of using extension method semantics to make dereferencing null-safe, but I find that used for limited scenarios like this, they make for more readable code. Also I know <code>Nullable.ToString</code> is already null-safe because the <code>Nullable</code> object itself isn't null, but that doesn't cater for parameters to the contained <code>ToString</code>, and I find that the explicitly named method indicates the null-safe intent.)</p>
 

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