How to get application memory usage as shown in Task Manager?

Presumably you're looking at the wrong column in "Task manager" or using the wrong property in Process class..

I guess you're looking for WorkingSet64 not PrivateMemorySize64. PrivateMemorySize64 is the amount of virtual memory allocated for the process, not the physical memory. For physical memory use WorkingSet64.

Also, you need to call process.Refresh() before accessing any of the dynamic properties in process class as it is heavily cached.

process.Refresh();
_data.MemoryUsed = (process.WorkingSet64).ConvertBytesToMegabytes().ToString(CultureInfo.InvariantCulture);