Syntax

try Statements... finally Statements... end;

try Statements... except Statements... end;

try Statements... except Handlers... end;

Description

The try keyword introduces a try-except statement or a try-finally statement. The two statements are related but serve different purposes.

The try-finally statement is used to manage memory and other resources. It performs the statements first in the try part of the statement, then in the finally part of the statement. The statements in the finally part are executed no matter how control leaves the try part: exception, returning from a subroutine with the Exit procedure, or exiting a loop with the Break procedure.

The try-except statement handles errors and exceptional conditions. It first performs the statements in the try part of the statement. If an exception is raised, control transfers to the except part of the statement, where Delphi searches for a matching exception handler. If the except part contains plain statements, Delphi executes those statements, and then control continues after the end of the try-except statement. If the except part contains one or more exception handlers (which start with the on directive), Delphi searches for a matching handler. If it cannot find a matching handler, control continues with the next try-except statement in the call stack.

Tips and Tricks

Example

// Copy a file.
procedure CopyFile(const DestFile, SourceFile: string);
const
  ReadFlags = fmOpenRead or fmShareDenyWrite;
resourcestring
  sSourceError = 'Cannot open source file: %s';
  sDestError = 'Cannot open destination file: %s';
  sCopyError = 'Cannot copy %1:s to %2:s: %0:s';
var
  DestStream, SourceStream: TStream;
begin
  DestStream := nil;
  SourceStream := nil;
  try
    try
      SourceStream := TFileStream.Create(SourceFile, ReadFlags);
      DestStream := TFileStream.Create(DestFile, fmCreate);
      DestStream.CopyFrom(SourceStream);
    except
    // EFCreateError and EFOpenError derive from EStreamError,
    // so check the derived classes first. Raise a new exception
    // that provides additional information. If any other kind
    // of exception is raised, the exception propagates to the
    // caller and its caller, until Delphi finds another exception
    // handler.
    on Ex: EFCreateError do
      raise EFCreateError.CreateFmt(sDestError, [Ex.Message]);
    on Ex: EFOpenError do
      raise EFOpenError.CreateFmt(sSourceError, [Ex.Message]);
    on Ex: EStreamError do
      raise EStreamError.CreateFmt(sCopyError,
                [Ex.Message, SourceFile, DestFile]);
    end;
  finally
    // Even if the try-except raises an exception, the finally
    // part executes, so the stream objects can be freed.
    DestStream.Free;
    SourceStream.Free;
  end;
end;

See Also