using System.Diagnostics.Debug; // for Assert
using System.Net.IPAddress; // for NetworkToHostOrder

Assert(0x12345678 == NetworkToHostOrder(   0x78563412)); // 4-byte
Assert(0x12340000 == NetworkToHostOrder(       0x3412)); // 4-byte
Assert(0x12000000 == NetworkToHostOrder(         0x12)); // 4-byte
Assert(    0x1234 == NetworkToHostOrder((short)0x3412)); // 2-byte
Assert(    0x1200 == NetworkToHostOrder(  (short)0x12)); // 2-byte

Assert(0x123456789abcde10 == NetworkToHostOrder(0x10debc9a78563412)); // 8-byte 

// using 'unchecked()' to convert large UInt32 values to negative Int32 values.
// See stackoverflow.com/questions/1131843/how-do-i-convert-uint-to-int-in-c 
// In this usage it'ss like the reinterpret cast because bits remain unchanged.
// Without unchecked, a runtime exception will be thrown 
// if the result becomes negative.
Assert( 0xFF1234FF == (UInt32)NetworkToHostOrder(unchecked((Int32)0xFF3412FF)));

using System.Diagnostics.Debug; // for Assert
using System.Net.IPAddress; // for NetworkToHostOrder

Assert(0x12345678 == NetworkToHostOrder(   0x78563412)); // 4-byte
Assert(0x12340000 == NetworkToHostOrder(       0x3412)); // 4-byte
Assert(0x12000000 == NetworkToHostOrder(         0x12)); // 4-byte
Assert(    0x1234 == NetworkToHostOrder((short)0x3412)); // 2-byte
Assert(    0x1200 == NetworkToHostOrder(  (short)0x12)); // 2-byte

Assert(0x123456789abcde10 == NetworkToHostOrder(0x10debc9a78563412)); // 8-byte 

// using unchecked to convert large UInt32 values to negative Int32 values.
// See stackoverflow.com/questions/1131843/how-do-i-convert-uint-to-int-in-c 
// unchecked in this usage is like the reinterpret cast because 
// it leaves the bits unchanged.
// Without unchecked, a runtime exception will be thrown 
// if the result becomes negative.
Assert( 0xFF1234FF == (UInt32)NetworkToHostOrder(unchecked((Int32)0xFF3412FF)));

using System.Diagnostics.Debug; // for Assert
using System.Net.IPAddress; // for NetworkToHostOrder

Assert(0x12345678 == NetworkToHostOrder(0x78563412)); // 4-byte
Assert(0x12340000 == NetworkToHostOrder(0x3412));     // 4-byte
Assert(0x12000000 == NetworkToHostOrder(0x12));       // 4-byte
Assert(0x1234 == NetworkToHostOrder((short)0x3412));  // 2-byte
Assert(0x1200 == NetworkToHostOrder((short)0x12));    // 2-byte
Assert(0x123456789abcde10 == NetworkToHostOrder(0x10debc9a78563412)); // 8-byte 

// using unchecked to convert large UInt32 values to negative Int32 values.
// See stackoverflow.com/questions/1131843/how-do-i-convert-uint-to-int-in-c 
// unchecked in this usage is like the reinterpret cast because 
// it leaves the bits unchanged.
Assert( 0xFF1234FF == (UInt32)NetworkToHostOrder(unchecked((Int32)0xFF3412FF)));

System.Diagnostics.Debug.Assert(0x12345678 == System.Net.IPAddress.NetworkToHostOrder(0x78563412)); // 4-byte
System.Diagnostics.Debug.Assert(0x12340000 == System.Net.IPAddress.NetworkToHostOrder(0x3412));     // 4-byte
System.Diagnostics.Debug.Assert(0x12000000 == System.Net.IPAddress.NetworkToHostOrder(0x12));       // 4-byte
System.Diagnostics.Debug.Assert(0x1234 == System.Net.IPAddress.NetworkToHostOrder((short)0x3412));  // 2-byte
System.Diagnostics.Debug.Assert(0x1200 == System.Net.IPAddress.NetworkToHostOrder((short)0x12));    // 2-byte
System.Diagnostics.Debug.Assert(0x123456789abcde10 == System.Net.IPAddress.NetworkToHostOrder(0x10debc9a78563412));     // 8-byte int

// Example using unchecked to convert large UInt32 values to negative Int32 values.
// See http://stackoverflow.com/questions/1131843/how-do-i-convert-uint-to-int-in-c 
// unchecked in this usage is like the reinterpret cast because it leaves the bits unchanged.
System.Diagnostics.Debug.Assert( 0xFF1234FF == (UInt32)System.Net.IPAddress.NetworkToHostOrder(unchecked((Int32)0xFF3412FF)));

System.Diagnostics.Debug.Assert(0x12345678 == System.Net.IPAddress.NetworkToHostOrder(0x78563412)); // 4-byte
            System.Diagnostics.Debug.Assert(0x12340000 == System.Net.IPAddress.NetworkToHostOrder(0x3412));     // 4-byte
            System.Diagnostics.Debug.Assert(0x12000000 == System.Net.IPAddress.NetworkToHostOrder(0x12));       // 4-byte
            System.Diagnostics.Debug.Assert(0x1234 == System.Net.IPAddress.NetworkToHostOrder((short)0x3412));  // 2-byte
            System.Diagnostics.Debug.Assert(0x1200 == System.Net.IPAddress.NetworkToHostOrder((short)0x12));    // 2-byte
            System.Diagnostics.Debug.Assert(0x123456789abcde10 == System.Net.IPAddress.NetworkToHostOrder(0x10debc9a78563412));     // 8-byte int

// Example using unchecked to convert large UInt32 values to negative Int32 values.
// See http://stackoverflow.com/questions/1131843/how-do-i-convert-uint-to-int-in-c 
// unchecked in this usage is like the reinterpret cast because it leaves the bits unchanged.
System.Diagnostics.Debug.Assert( 0xFF1234FF == (UInt32)System.Net.IPAddress.NetworkToHostOrder(unchecked((Int32)0xFF3412FF)));

System.Diagnostics.Debug.Assert(0x12345678 == System.Net.IPAddress.NetworkToHostOrder(0x78563412)); // 4-byte
            System.Diagnostics.Debug.Assert(0x12340000 == System.Net.IPAddress.NetworkToHostOrder(0x3412));     // 4-byte
            System.Diagnostics.Debug.Assert(0x12000000 == System.Net.IPAddress.NetworkToHostOrder(0x12));       // 4-byte
            System.Diagnostics.Debug.Assert(0x1234 == System.Net.IPAddress.NetworkToHostOrder((short)0x3412));  // 2-byte
            System.Diagnostics.Debug.Assert(0x1200 == System.Net.IPAddress.NetworkToHostOrder((short)0x12));    // 2-byte
            System.Diagnostics.Debug.Assert(0x123456789abcde10 == System.Net.IPAddress.NetworkToHostOrder(0x10debc9a78563412));     // 8-byte int

This shows how to implement simple byte swapping in C#.

It's shown in the context of assert statements instead of print statements to show the expected results.

Interesting post  http://bytes.com/topic/c-sharp/answers/506861-littleendian-bigendian compared performance of various methods. mask-shift-or performed best.

byte swapping using System.Net.IPAddress.NetworkToHostOrder

C#