Set text of a control of a child-window in WPF

How can i set a text of a textblock of a c开发者_运维问答hild window. childWindowname.textblockName.Text = "" doesn't have any effect.

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Odd. I have used Text from codebehind to change the text in my window without any problems. The only difference is that I had a public method in my child window that I called, and that set textblockname.Text.

e.g.

public void SetTextBlockMessage(string message)
{
  textblockname.Text = message;
}