Delphi TDictionary迭代

Question

I have a function where I store some key- value pairs and when I iterate them I get this error twice: [dcc32 Error] App.pas(137): E2149 Class does not have a default property. Here is part of my code:

function BuildString: string;
var
  i: Integer;
  requestContent: TDictionary<string, string>;
  request: TStringBuilder;
begin
  requestContent := TDictionary<string, string>.Create();

  try
    // add some key-value pairs
    request :=  TStringBuilder.Create;
    try
      for i := 0 to requestContent.Count - 1 do
      begin
        // here I get the errors
        request.Append(requestContent.Keys[i] + '=' +
          TIdURI.URLEncode(requestContent.Values[i]) + '&');
      end;

      Result := request.ToString;
      Result := Result.Substring(0, Result.Length - 1); //remove the last '&'
    finally
      request.Free;
    end; 
  finally
    requestContent.Free;
  end;
end;

I need to collect the information from each item in the dictionary. How can I fix it?

Answer 1

The Keys and Values properties of your dictionary class are of type TDictionary<string, string>.TKeyCollection and TDictionary<string, string>.TValueCollection respectively. These classes are derived from TEnumerable<T> and cannot be iterated by index. You can however iterate over Keys , or indeed Values , not that doing the latter would be of much use to you.

If you iterated over Keys your code might look like this:

var
  Key: string;
....
for Key in requestContent.Keys do
  request.Append(Key + '=' + TIdURI.URLEncode(requestContent[Key]) + '&');

This however is inefficient. Since you know that you want both key and matching value, you can use the dictionary's iterator:

var 
  Item: TPair<string, string>; 
....
for Item in requestContent do 
  request.Append(Item.Key + '=' + TIdURI.URLEncode(Item.Value) + '&');

The pair iterator is more efficient than the first variant above. This is because the implementation details mean that the pair iterator is able to iterate the dictionary without:

1. Calculating hash codes for each key, and
2. Performing linear probing when hash codes collide.