// C# does not provide the ability to set a field within a byte to a value.
// It has the ability to set individual bits, but if you are accessing hardware
// you may need to set fields within bytes - no way jose. Until now!

/// <summary>
/// SetField is used to set a certain set of bits to the value specified.
/// </summary>
/// <param name="input">byte in which to set the field</param>
/// <param name="value">value to set field to</param>
/// <param name="location">bit number within byte where field begins</param>
/// <param name="noBits">field size</param>
private void SetField(ref long input, long value, byte location, int noBits)
{
	int size = 1;
	if ((location > 31) || (location < 0))
		throw new Exception("location out of range: " + location);
	if ((noBits > 31) || (noBits <= 0))
		throw new Exception("noBits out of range: " + noBits);
	if (noBits > 1) size = size << noBits;
	if ((value < 0) || (value > size))
		throw new Exception("value out of range: " + value);
	value = value << location;
	if (value < 0)
		throw new Exception("value out of range after shifting to field location: " + value);
	input = (input | value);
}

Test:

using System;

namespace testApp
{
	/// <summary>
	/// Summary description for Class1.
	/// </summary>
	class Class1
	{
		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main(string[] args)
		{
			Class1 t = new Class1();
			long i = 2; // 
			Console.WriteLine(t.ConvertByteToBinaryString(i));
			t.SetField(ref i,5,8,4);
			Console.WriteLine(t.ConvertByteToBinaryString(i));
		}
		/// <summary>
		/// SetField is used to set a certain set of bits to the value specified.
		/// </summary>
		/// <param name="input">byte in which to set the field</param>
		/// <param name="value">value to set field to</param>
		/// <param name="location">bit number within byte where field begins</param>
		/// <param name="noBits">field size</param>
		private void SetField(ref long input, long value, byte location, int noBits)
		{
			int size = 1;
			if ((location > 31) || (location < 0))
				throw new Exception("location out of range: " + location);
			if ((noBits > 31) || (noBits <= 0))
				throw new Exception("noBits out of range: " + noBits);
			if (noBits > 1) size = size << noBits;
			if ((value < 0) || (value > size))
				throw new Exception("value out of range: " + value);
			value = value << location;
			if (value < 0)
				throw new Exception("value out of range after shifting to field location: " + value);
			input = (input | value);
		}
		/// <summary>
		/// Format integer as a binary string with spaces between every 4 bits
		/// </summary>
		/// <param name="input">integer to format</param>
		/// <returns>string of the form "0000 0000 0000 0000 0000 0110 0001 1000"</returns>
		private string ConvertByteToBinaryString(long input)
		{
			string result = null;
			string bitstring = null;
			for(int i=31;i>=0;i--)
			{
				if (((input >> i) & 1) == 1) result = "1";
				else result = "0";
				bitstring += result;
				if ((i>0) && ((i)%4)==0)
				{
					bitstring += " ";
				}
			}
			return bitstring;
		}
	}
}

Output from test:
C:\>testApp.exe
0000 0000 0000 0000 0000 0000 0000 0010
0000 0000 0000 0000 0000 0101 0000 0010
C:\>