If fromBase is 16, you can prefix the number specified by the value parameter with "0x" or "0X".

Because the byte data type supports unsigned values only, the Convert.ToByte(string, int) method assumes that value is expressed using unsigned binary representation. In other words, all eight bits are used to represent the numeric value, and a sign bit is absent. As a result, it is possible to write code in which a signed byte value that is out of the range of the byte data type is converted to a byte value without the method throwing an exception. The following example converts sbyte.MinValue to its hexadecimal string representation, and then calls the Convert.ToByte(string, int) method. Instead of throwing an exception, the method displays the message, "0x80 converts to 128."

code reference: System.Convert.BaseConversion#3

When performing binary operations or numeric conversions, it is always the responsibility of the developer to verify that a method or operator is using the appropriate numeric representation to interpret a particular value. The following example illustrates one technique for ensuring that the method does not inappropriately use unsigned binary representation when it converts a hexadecimal string representation to a byte value. The example determines whether a value represents a signed or an unsigned integer while it is converting that value to its string representation. When the example converts the value back to a byte value, it checks whether the original value was a signed integer. If so, and if its high-order bit is set (which indicates that the value is negative and that it uses two's complement instead of unsigned binary representation), the method throws an exception.

code reference: System.Convert.BaseConversion#4

Namespace: System
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0