To reduce nested IF:

1/ early return;

2/ compound expression (short-circuit aware)

So, your example may be refactored like this:

if( FileExists(file) && SomeTest(contents = OpenFile(file)) )
{
    DoSomething(contents);
    return;
}
DefaultAction();

To reduce nested IF:

1/ early return;

2/ compound expression (short-circuit aware)

So, your example may be refactored like this:

if( FileExists(file) && SomeTest(contents = OpenFile(file)) )
{
    DoSomething(contents);
    return;
}
DefaultAction();

To reduce nested IF:

1/ early return;

2/ compound expression (short-circuit aware)

So, your example may be refactored like this:

if( FileExists(file) && SomeTest(contents = OpenFile(file)) )
{
    DoSomething(contents);
    return;
}
DefaultAction();

--dq

To reduce nested IF:

1/ early return;

2/ compound expression (short-circuit aware)

So, your example may be refactored like this:

if( FileExists(file) && SomeTest(contents = OpenFile(file)) )
{
    DoSomething(contents);
    return;
}
DefaultAction();