List objects are one-based

Properties

PC0004 Error Design Code Fix Ignore Obsolete

Developers coming from C#, JavaScript, or most other languages expect zero-based indexing. In AL, List is one-based: valid indices run from 1 to Count().

Lists are 1-based indexed, that is, the indexing of a List begins with 1.

— List Data Type on Microsoft Learn

Passing 0 to Get or starting a for loop at 0 compiles without errors but raises an index-out-of-range error at runtime. Start indexing from 1.

Example

procedure ProcessList()
var
    MyList: List of [Integer];
    i: Integer;
begin
    MyList.Add(10);
    MyList.Add(20);
    i := MyList.Get(0); // List objects are one-based [PC0004]
end;

Use one-based indexing:

procedure ProcessList()
var
    MyList: List of [Integer];
    i: Integer;
begin
    MyList.Add(10);
    MyList.Add(20);
    i := MyList.Get(1);
end;

The diagnostic is also raised when a for loop iterating over a List starts at 0:

procedure IterateList()
var
    MyList: List of [Text];
    i: Integer;
begin
    MyList.Add('First');
    MyList.Add('Second');
    for i := 0 to MyList.Count() do // List objects are one-based [PC0004]
        Message(MyList.Get(i));
end;

Start the loop at 1:

procedure IterateList()
var
    MyList: List of [Text];
    i: Integer;
begin
    MyList.Add('First');
    MyList.Add('Second');
    for i := 1 to MyList.Count() do
        Message(MyList.Get(i));
end;