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If it is WPF, in XAML just use the following:
HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
If it is Winforms, just change the option Dock
for Fill
:
tabControl1.Dock = DockStyle.Fill;
Or change the property directly in the panel:
Is wpf? If it is passes the XAML of this form.
– Augusto Vasques