How to get the amount of memory used by an application

You can try GC.GetTotalMemory:

It Retrieves the number of bytes currently thought to be allocated. A parameter indicates whether this method can wait a short interval > before returning, to allow the system to collect garbage and finalize > objects.

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or

using System.Diagnostics;

Process currentProc = Process.GetCurrentProcess();

Once you have a reference to the current process, you can determine its memory usage by reading the PrivateMemorySize64 property.

long memoryUsed = currentProc.PrivateMemorySize64;

You can use the following function (The true parameter tells the GC to perform a collection first):

long memory = GC.GetTotalMemory(true);