How do I call native windows APIs from managed c# code?

I want to use the following native APIs:

NtOpenProcess 
 NtOpenThread 
 NtReadVirtualMemory 
 NtWriteVirtualMemory 
 PsCreateSystemThread 
 KiAttachProcess
 

Is it possible to call these APIs from managed C# code? If so, can someone provide an example of how this would be done?

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