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    <p>Been a while (years) since I've looked at assembly, and I know close to nothing about MSIL and JIT, so it would be a nice exercise - couldn't resist, so here's just a bit of, possibly redundant, empirical data. Does the <code>IndexOf</code> overload get inlined?</p> <p>Here's a tiny Console app:</p> <pre><code>class Program { static void Main(string[] args) { "hello".Contains("hell"); } } </code></pre> <p>The JIT generates this in an optimized Release build, Any CPU, running in 32 bit. I've shortened the addresses, and removed some irrelevant lines:</p> <pre><code>--- ...\Program.cs "hello".Contains("hell"); [snip] 17 mov ecx,dword ptr ds:[0320219Ch] ; pointer to "hello" 1d mov edx,dword ptr ds:[032021A0h] ; pointer to "hell" 23 cmp dword ptr [ecx],ecx 25 call 680A6A6C ; String.Contains() [snip] </code></pre> <p>The <code>call</code> at 0x00000025 goes here:</p> <p><strong>String.Contains</strong></p> <pre><code>00 push 0 ; startIndex = 0 02 push dword ptr [ecx+4] ; count = this.Length (second DWORD of String) 05 push 4 ; comparisonType = StringComparison.Ordinal 07 call FF9655A4 ; String.IndexOf() 0c test eax,eax 0e setge al ; if (... &gt;= 0) 11 movzx eax,al 14 ret </code></pre> <p>Sure enough, it seems to call, directly, the final <code>String.IndexOf</code> overload with four arguments: three <code>push</code>ed; one in <code>edx</code> (<code>value</code>: "hell"); <code>this</code> ("hello") in <code>ecx</code>. To confirm, this is where the <code>call</code> at 0x00000005 goes:</p> <pre><code>00 push ebp 01 mov ebp,esp 03 push edi 04 push esi 05 push ebx 06 mov esi,ecx ; this ("hello") 08 mov edi,edx ; value ("hell") 0a mov ebx,dword ptr [ebp+10h] 0d test edi,edi ; if (value == null) 0f je 00A374D0 15 test ebx,ebx ; if (startIndex &lt; 0) 17 jl 00A374FB 1d cmp dword ptr [esi+4],ebx ; if (startIndex &gt; this.Length) 20 jl 00A374FB 26 cmp dword ptr [ebp+0Ch],0 ; if (count &lt; 0) 2a jl 00A3753F [snip] </code></pre> <p>... which would be the body of:</p> <pre><code>public int IndexOf(string value, int startIndex, int count, StringComparison comparisonType) { if (value == null) throw new ArgumentNullException("value"); if (startIndex &lt; 0 || startIndex &gt; this.Length) throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index")); if (count &lt; 0 || startIndex &gt; this.Length - count) throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_Count")); ... } </code></pre>
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    1. POWhy doesn't String.Contains call the final overload directly?
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    1. POWhy doesn't String.Contains call the final overload directly?
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    1. COWow, thanks for effort! It seems that the performance difference is negligible (for non-case sensitive searches) and so it comes down to a design decision. I guess it really might just be a case of having to avoid two code changes should the 4 overload signature ever change (however unlikely).
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    2. COOf course, right now I realized that the call stack could tell me this. But fun to have my first real look into the JIT output, nonetheless.
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    3. COI haven't delved into Assembly to any real degree so your answer has been very interesting to go through.
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