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Pitfalls reading from the Registry (ProxyEnable) (Views: 100)


Problem/Question/Abstract:

Pitfalls reading from the Registry (ProxyEnable)

Answer:

The following has been verified for Delphi 3. It may be slightly different for Delphi 5.
In a project I had to read the Boolean field “ProxyEnable” from the registry. It is stored in

HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings

On the systems that I work with (NT 4.0/ sp 5, NT 4.0/ sp 6, Win 2000/ sp 1) I checked and determined that this value was stored as a REG_DWORD. 0 meant false, 1 meant true. So I simply coded as shown in 1)

Pitfall 1): Unexpected Entry Type

On some customer machines, the value ProxyEnable was stored as a REG_BINARY. Reading a REG_BINARY with ReadInteger() causes an exception – and to make things worse, exception handling was not yet in place since it was executed during the initialization of my application.

So I changed my code to 2) and hoped it would work.


Pitfall 2): Read the exact length

This was a surprising one. When you have a registry value of type REG_BINARY that is 4 bytes long and you try reading it with

ReadBinaryData("MyKey", myBooleanVar, 1);

… then you will experience another exception. The correct code is as shown in part 3.

// Version 1:
// raises an exception because of the unexpected entry type

ProxyEnabled := ReadBool(sProxyEnable);

// Version 2:
// detect type with GetDataType(),
// but not size... raises an exception if the size is wrong

case GetDataType(sProxyEnable) of
  rdInteger:
    ProxyEnabled := ReadBool(sProxyEnable);
  rdBinary:
    ReadBinaryData(sProxyEnable, ProxyEnabled, 1);
  // other types..
end;

// Version 3:
// detect type and size (GetDataSize())! Works so far.

case GetDataType(sProxyEnable) of
  rdInteger:
    ProxyEnabled := ReadBool(sProxyEnable);
  rdBinary:
    ReadBinaryData(sProxyEnable, ProxyEnabled, GetDataSize(sProxyEnable));
  // other types..
end;

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