How can I write on another process memory?

Question

I have an address that I would like to modify. I have the process. I have the new value. So now what?

// My Process
var p = Process.GetProcessesByName("ePSXe").FirstOrDefault();

// Address
var addr = 0x00A66E11;

// Value
var val = 0x63;

How can I write 0x63 (99) to this address on another process memory?

Answer 1

@Harvey, from your answer I dug up and found a lot:

Open, Close and Write signatures:

[DllImport("kernel32.dll")]
static extern IntPtr OpenProcess(ProcessAccessFlags dwDesiredAccess, [MarshalAs(UnmanagedType.Bool)] bool bInheritHandle, int dwProcessId);

[DllImport("kernel32.dll", SetLastError = true)]
static extern bool WriteProcessMemory(IntPtr hProcess, IntPtr lpBaseAddress, byte[] lpBuffer, uint nSize, out int lpNumberOfBytesWritten);

[DllImport("kernel32.dll")]
public static extern Int32 CloseHandle(IntPtr hProcess);

Flags:

[Flags]
public enum ProcessAccessFlags : uint
{
    All = 0x001F0FFF,
    Terminate = 0x00000001,
    CreateThread = 0x00000002,
    VMOperation = 0x00000008,
    VMRead = 0x00000010,
    VMWrite = 0x00000020,
    DupHandle = 0x00000040,
    SetInformation = 0x00000200,
    QueryInformation = 0x00000400,
    Synchronize = 0x00100000
}

Make my life easier method:

public static void WriteMem(Process p, int address, long v)
{
    var hProc = OpenProcess(ProcessAccessFlags.All, false, (int)p.Id);
    var val = new byte[] { (byte)v };

    int wtf = 0;
    WriteProcessMemory(hProc, new IntPtr(address), val, (UInt32)val.LongLength, out wtf);

    CloseHandle(hProc);
}

Writing into another process memory:

static void Main(string[] args)
{
    var p = Process.GetProcessesByName("ePSXe").FirstOrDefault();

    WriteMem(p, 0x00A66DB9, 99);
}