• Articolo

Convert.ToByte Method (String, Int32)

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Converts the string representation of a number in a specified base to an equivalent 8-bit unsigned integer.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

Syntax

'Declaration
Public Shared Function ToByte ( _
    value As String, _
    fromBase As Integer _
) As Byte

public static byte ToByte(
    string value,
    int fromBase
)

Parameters

  • fromBase
    Type: System.Int32
    The base of the number in value, which must be 2, 8, 10, or 16.

Return Value

Type: System.Byte
An 8-bit unsigned integer equivalent to the number in value.
-or-
Zero if value is nulla null reference (Nothing in Visual Basic).

Exceptions

Exception Condition
ArgumentException

fromBase is not 2, 8, 10, or 16.

-or-

value, which represents a non-base 10 unsigned number, is prefixed with a negative sign.
FormatException

value contains a character that is not a valid digit in the base specified by fromBase. The exception message indicates that there are no digits to convert if the first character in value is invalid; otherwise, the message indicates that value contains invalid trailing characters.
OverflowException

value, which represents a base 10 unsigned number, is prefixed with a negative sign.

-or-

The return value is less than Byte.MinValue or larger than Byte.MaxValue.

Remarks

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 ToByte(String, Int32) 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 MinValue to its hexadecimal string representation and then calls the ToByte(String, Int32) method. Rather than throwing an exception, the method displays the message, "0x80 converts to 128."

' Create a hexadecimal value out of range of the Byte type.
Dim value As String = SByte.MinValue.ToString("X")
' Convert it back to a number.
Try
   Dim number As Byte = Convert.ToByte(value, 16)
   outputBlock.Text += String.Format("0x{0} converts to {1}.", value, number) & vbCrLf
Catch e As OverflowException
   outputBlock.Text += String.Format("Unable to convert '0x{0}' to a byte.", value) & vbCrLf
End Try

// Create a hexadecimal value out of range of the Byte type.
string value = SByte.MinValue.ToString("X");
// Convert it back to a number.
try
{
   byte number = Convert.ToByte(value, 16);
   outputBlock.Text += String.Format("0x{0} converts to {1}.", value, number) + "\n";
}
catch (OverflowException)
{
   outputBlock.Text += String.Format("Unable to convert '0x{0}' to a byte.", value) + "\n";
}

When performing binary operations or numeric conversions, it is always the responsibility of the developer to verify that a method or operator uses 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 converting a hexadecimal string representation to a Byte value. It determines whether a value represents a signed or an unsigned integer at the same time that it converts that value to its string representation. When converting the value back to a Byte value, the example checks whether the original value was a signed integer. If so, and its high-order bit is set (which indicates that the value is negative and that it uses two's complement rather than unsigned binary representation), the method throws an exception.

' Create a negative hexadecimal value out of range of the Byte type.
Dim sourceNumber As SByte = SByte.MinValue
Dim isSigned As Boolean = Math.Sign(SByte.MinValue) = -1
Dim value As String = sourceNumber.ToString("X")
Dim targetNumber As Byte
Try
   targetNumber = Convert.ToByte(value, 16)
   If isSigned And ((targetNumber And &H80) <> 0) Then
      Throw New OverflowException()
   Else
      outputBlock.Text += String.Format("0x{0} converts to {1}.", value, targetNumber) & vbCrLf
   End If
Catch e As OverflowException
   outputBlock.Text += String.Format("Unable to convert '0x{0}' to an unsigned byte.", value) & vbCrLf
End Try
' Displays the following:
'    Unable to convert '0x80' to an unsigned byte.     

// Create a negative hexadecimal value out of range of the Byte type.
sbyte sourceNumber = SByte.MinValue;
bool isSigned = Math.Sign((sbyte)sourceNumber.GetType().GetField("MinValue").GetValue(null)) == -1;
string value = sourceNumber.ToString("X");
byte targetNumber;
try
{
   targetNumber = Convert.ToByte(value, 16);
   if (isSigned && ((targetNumber & 0x80) != 0))
      throw new OverflowException();
   else
      outputBlock.Text += String.Format("0x{0} converts to {1}.", value, targetNumber) + "\n";
}
catch (OverflowException)
{
   outputBlock.Text += String.Format("Unable to convert '0x{0}' to an unsigned byte.", value) + "\n";
}
// Displays the following:
//    Unable to convert '0x80' to an unsigned byte.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.