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jimfred on 08/15/09

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Last Edited at 08/15/09 07:31pm

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C# structs, byte swapping for conversion from big to little endian using reflection to enumerate members

/ Published in: C#

This example uses a Swap member to enumerate the struct’s fields and swap multi-byte fields. It doesn’t traverse nested structs.

It would be nice if dot.net has something equivalent to ‘#pragma endian big’ as seen in some compilers. There might be a way to leverage marshaling to automatically endianize multi-byte values.

This example uses:
* System.Net.IPAddress.NetworkToHostOrder for byte swapping
* unchecked() to convert a value of, say, 0 × 87654321 to a Int32 without a runtime error
* foreach (System.Reflection.FieldInfo field in typeof(MyStruct).GetFields()) to enumerate structure members
* TypeCode typeCode = Type.GetTypeCode(fieldValue.GetType()); to get the underlying type enumeration (typeof of verbose).
* SetValue and GetValue to access struct members.
* Boxed object of struct - this was necessary only because SetValue didn’t work on structs. It failed quietly.

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public struct MyStruct
{
    public UInt32 u4;
    public int  s4;
    public UInt16 u2;
    public short  s2;
    public UInt64 u8;
    public Int64  s8;
               
    // byte-swap members of this struct.
    public MyStruct Swap()
    {
        // The copy is made becuase SetValue won't work on a struct.
        // Boxing was used because SetValue works on classes/objects.
        // Unfortunately, it results in 2 copy operations.
        object thisBoxed = this; // Why make a copy?

// System.Net.IPAddress.NetworkToHostOrder is used to perform byte swapping.
        // To convert unsigned to signed, 'unchecked()' was used.
        // See http://stackoverflow.com/questions/1131843/how-do-i-convert-uint-to-int-in-c

// Enumerate each structure field using reflection.
        foreach (var field in typeof(MyStruct).GetFields())
        {
            // field.Name has the field's name.

object fieldValue = field.GetValue(thisBoxed); // Get value

// Get the TypeCode enumeration. Multiple types get mapped to a common typecode.
            TypeCode typeCode = Type.GetTypeCode(fieldValue.GetType());
            
            switch (typeCode)
            {
                case TypeCode.Int32:
                {
                    Int32 i = System.Net.IPAddress.NetworkToHostOrder((Int32)fieldValue);
                    field.SetValue(thisBoxed, (Int32)i);
                    break;
                }
                case TypeCode.UInt32:
                {
                    Int32 i = System.Net.IPAddress.NetworkToHostOrder(unchecked((Int32)(UInt32)fieldValue));
                    field.SetValue(thisBoxed, (UInt32)i);
                    break;
                }
                case TypeCode.Int16:
                {
                    Int16 i = System.Net.IPAddress.NetworkToHostOrder((Int16)fieldValue);
                    field.SetValue(thisBoxed, (Int16)i);
                    break;
                }
                case TypeCode.UInt16:
                {
                    Int16 i = System.Net.IPAddress.NetworkToHostOrder(unchecked((Int16)(UInt16)fieldValue));
                    field.SetValue(thisBoxed, (UInt16)i);
                    break;
                }
                case TypeCode.Int64:
                {
                    Int64 i = System.Net.IPAddress.NetworkToHostOrder((Int64)fieldValue);
                    field.SetValue(thisBoxed, i);
                    break;
                }
                case TypeCode.UInt64:
                {
                    Int64 i = System.Net.IPAddress.NetworkToHostOrder(unchecked((Int64)(UInt64)fieldValue));
                    field.SetValue(thisBoxed, (UInt64)i);
                    break;
                }
                default:
                {
                    System.Diagnostics.Debug.Fail("No conversion provided for this type");
                    break;
                }
            }; // switch

} // foreach
        return (MyStruct)thisBoxed;
    } // Swap
}; // MyStruct

// Usage example:
    MyStruct s = new MyStruct();
    
    s.u4 = 0x78563412; 
    s.s4 = 0x21436587;
    s.u2 = 0x2143;
    s.s2 = 0x3412;
    s.u8 = 0xFF007856341200FF;
    s.s8 = 0x7700785634120077;

s = s.Swap();

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