C# OutOfMemoryException and StringBuilder

We had an interesting OutOfMemoryException this week. When creating really long strings and adding them to a StringBuilder, we get the OutOfMemoryException way before we really run out of memory. Come to find out, StringBuilder is searching for contiguous memory and may not find that long before you run out of real memory.

To help find your upper limit of memory use, .Net provides the friendly MemoryFailPoint as shown below:

using System;
using System.Runtime;
using System.Text;
namespace PlayingAround {
    class Memory {
        private static void Main() {
            MemoryFailPoint memFailPoint;
            StringBuilder sb = new StringBuilder("1234567890");
            
            for (int i = 0; i < 100; i++) {
                Console.Out.WriteLine("");
                try
                {
                    int aboutToGetMemoryInBytes = sb.Length*5;
                    Console.Out.WriteLine("aboutToGetMemoryInBytes = " + aboutToGetMemoryInBytes.ToString("#,##0")); 
                   int aboutToGetMemoryInMegaBytes = (int)(1 + (aboutToGetMemoryInBytes >> 20)); //round up to the next megabyte
                   Console.Out.WriteLine("aboutToGetMemoryInMegaBytes = " + aboutToGetMemoryInMegaBytes.ToString("#,##0"));
                    using (memFailPoint = new MemoryFailPoint(aboutToGetMemoryInMegaBytes)) {
                        Console.Out.WriteLine("GC.GetTotalMemory(true) = " + GC.GetTotalMemory(true).ToString("#,##0"));
                        Console.Out.WriteLine("sb.Length = " + sb.Length.ToString("#,##0"));
                        //Console.Out.WriteLine(System.Environment.WorkingSet.ToString());
                        sb.Append(sb.ToString());
                    }
                }
                catch (InsufficientMemoryException) {
                    Console.Out.WriteLine("InsufficientMemoryException.");
                    var maxMem = GC.GetTotalMemory(true);
                    Console.Out.WriteLine("maxMem = " + maxMem);
                    break;
                } catch (OutOfMemoryException) {
                    Console.Out.WriteLine("OutOfMemoryException.");
                    break;
                }
            }
            Console.Out.Write("press return to exit.");
            Console.In.ReadLine();
        }
    }
}