What are the performance best practices in C#?

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**Performance tips** for faster C# code. **1. Use Span for string operations:** ```csharp // Bad string sub = str.Substring(1, 5); // Allocation // Good Span span = str.AsSpan().Slice(1, 5); // No allocation ``` **2. Use StringBuilder for loops:** ```csharp // Bad string s = ""; for (int i = 0; i < 1000; i++) s += i; // 1000 allocations // Good var sb = new StringBuilder(); for (int i = 0; i < 1000; i++) sb.Append(i); // 1 allocation ``` **3. Use struct for small types:** ```csharp // Bad class Point { public int X, Y; } // Heap allocation // Good struct Point { public int X, Y; } // Stack allocation ``` **4. Use object pooling:** ```csharp var pool = ArrayPool.Shared; byte[] buffer = pool.Rent(1024); try { /* use buffer */ } finally { pool.Return(buffer); } ``` **5. Use frozen collections:** ```csharp var frozen = dict.ToFrozenDictionary(); // Optimized reads ```

C#: What are the performance best practices in C#?