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C# If Versus Switch PerformanceCompare the performance of if and switch statements. Describe how to select the fastest one.
If, switch. In C# programs, if and switch have different performance. Knowing some of the performance details is useful—some programs can benefit.
Comparison notes. We compare the performance of if-statements and switch-statements. Choosing the best statement type depends on the data we are testing.
If
Switch
Benchmark. This program declares 2 methods that are tested. The results of the 2 methods on all inputs are equivalent—they have the same effects.
Version 1 This version is called IsValidIf—it implements a selection with several if-statements.
Version 2 This version of the code is called IsValidSwitch—it implements a selection as a switch.
Result In .NET 5 for Linux in 2021, the switch statement method (version 2) is faster. Switch is a performance improvement.
C# program that times if and switch methods
using System; using System.Diagnostics; class Program { static bool IsValidIf(int i) { // Uses if-expressions to implement selection statement. if (i == 0 || i == 1) { return true; } if (i == 2 || i == 3) { return false; } if (i == 4 || i == 5) { return true; } return false; } static bool IsValidSwitch(int i) { // Implements a selection statement with a switch. switch (i) { case 0: case 1: return true; case 2: case 3: return false; case 4: case 5: return true; default: return false; } } const int _max = 100000000; static void Main() { bool b; var s1 = Stopwatch.StartNew(); // Version 1: use if-statement. for (int i = 0; i < _max; i++) { b = IsValidIf(i); } s1.Stop(); var s2 = Stopwatch.StartNew(); // Version 2: use switch-statement. for (int i = 0; i < _max; i++) { b = IsValidSwitch(i); } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns")); } }
3.83 ns [if] 2.88 ns [switch]
Benchmark, fast path. This benchmark tests 2 implementations. The methods receive the value zero 60% of the time. They receive the value one 40% of the time.
Version 1 This version of the code uses a switch statement. All of the cases are tested with equal priority (none is tested first).
Version 2 This code uses an if-else construct. The first int tested is 0, which gives it priority over other tests.
Result In .NET 5 in 2021, Method 2, which uses if, is faster. Sometimes using if-statements is faster.
C# program that benchmarks switch, if, fast path
using System; using System.Diagnostics; class Program { const int _max = 100000000; static void Main() { Method1(0); // JIT. Method2(0); // JIT. var s1 = Stopwatch.StartNew(); // Version 1: use switch. for (int i = 0; i < _max; i++) { Method1(0); Method1(0); Method1(0); Method1(0); Method1(0); Method1(0); Method1(1); Method1(1); Method1(1); Method1(1); } s1.Stop(); var s2 = Stopwatch.StartNew(); // Version 2: use if, fast path. for (int i = 0; i < _max; i++) { Method2(0); Method2(0); Method2(0); Method2(0); Method2(0); Method2(0); Method2(1); Method2(1); Method2(1); Method2(1); } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns")); } static int Method1(int val) { switch (val) { case 0: { return 1; } case 1: { return 3; } default: { throw new Exception(); } } } static int Method2(int val) { if (val == 0) { return 1; } if (val == 1) { return 3; } throw new Exception(); } }
18.89 ns [switch] 0.53 ns [if]
Notes, switch slower. It is tempting to think that a switch is always faster than an equivalent if-statement. However, this is not true.
And A situation where the switch is slower is when the actual runtime of the program has a very skewed distribution of inputs.
Benchmark
So If the input is almost always a specific value, then using an if-statement to test for that value may be faster.
A summary. Sometimes a switch statement is slower than an if-statement. Using frequency heuristics, you can optimize a fast path with an if-statement in many programs.
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