From Xojo Documentation

Property (As String )
aRuntimeException.Message = newStringValue
StringValue = aRuntimeException.Message

Supported for all project types and targets.

Used to contain descriptive text to display when the runtime exception is encountered.

Sample Code

The following example displays a message box if, for example, you try to create more than one KeyChainItem for the same application.

Dim newItem As KeyChainItem
If System.KeyChainCount > 0 Then
newItem = New KeyChainItem
// Indicate the name of the application
newItem.ServiceName = "MyApplication"

// Create a new keychain item for the application and assign the password
System.KeyChain.AddPassword(newItem, "SecretPassword")
MsgBox("You don't have a key chain.")
End If
Exception err As KeyChainException
MsgBox(err.Message + ". Error Code: " + Str(err.ErrorNumber))