Hide Windows form from WPF UserControl

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C#

WPF

Hi to everyone.
Im "dummy" in wpf but i used a hostControl to display Wpf form in login Form. When i click Login button it couses MDI form to open but i can not Hide Login Form.
i use in wpf this.Visibility = Hidden; but the windows form is not hidden.
please help me.
best regard!

OP's additional information moved from non-solution below
in MDI load i used

FormCollection fc = Application.OpenForms;
foreach (Form frm in fc)
{
if(frm.Equals("frmLogin"))
frm.Hide();
}