tui-cs/Terminal.Gui · error · NullReferenceException

Target

Error message

Target

What it means

This error is thrown by PosView.ToString() when the Target property is null. PosView represents a layout position anchored to a side (Left, Right, Top, Bottom) of a target View. ToString() formats the position as 'View(Side=...,Target=...)'. If Target is null -- which should not happen in normal usage since PosView is created via Pos.Left(view), Pos.Right(view), etc. -- ToString throws a NullReferenceException rather than producing misleading output.

Source

Thrown at Terminal.Gui/ViewBase/Layout/PosView.cs:49

    /// <summary>
    ///     Gets the View the position is anchored to.
    /// </summary>
    public View Target { get; }

    /// <summary>
    ///     Gets the side of the View the position is anchored to.
    /// </summary>
    public Side Side { get; }

    /// <inheritdoc/>
    public override string ToString ()
    {
        var sideString = Side.ToString ();

        if (Target == null)
        {
            throw new NullReferenceException (nameof (Target));
        }

        return $"View(Side={sideString},Target={Target})";
    }

    internal override int GetAnchor (int size) =>
        Side switch
        {
            Side.Left => Target.Frame.X,
            Side.Top => Target.Frame.Y,
            Side.Right => Target.Frame.Right,
            Side.Bottom => Target.Frame.Bottom,
            _ => 0
        };

    internal override bool ReferencesOtherViews () => true;

    /// <inheritdoc/>

View on GitHub (pinned to 2e47b11478)

Solutions

1. Ensure PosView instances are always created via Pos.Left(view)/Pos.Right(view)/Pos.Top(view)/Pos.Bottom(view) which guarantee a non-null Target.
2. If deserializing, validate Target is set before use.
3. Avoid disposing or nullifying views that are referenced by PosView instances.
4. If this occurs during debugging, inspect the Pos hierarchy to find the partially-constructed PosView.

Example fix

// before -- a PosView with null Target throws on ToString
PosView pos = new PosView(null, Side.Left);
Console.WriteLine(pos.ToString());

// after -- always create PosView via the Pos factory methods
Pos pos = Pos.Left(myView);
Console.WriteLine(pos.ToString());

// Always create PosView via factory methods which guarantee non-null Target
Pos p = Pos.Left(myView); // Target is guaranteed set

bool HasValidTarget(PosView p) => p.Target is not null;

try { Console.WriteLine(pos.ToString()); }
catch (NullReferenceException ex) when (ex.Message == "Target")
{ /* PosView was created without a Target -- use factory methods */ }

When it happens

Trigger: Thrown at PosView.cs:49 when ToString() is called on a PosView instance whose Target property is null. PosView is normally created via the Pos.Left(view)/Pos.Right(view)/Pos.Top(view)/Pos.Bottom(view) factory methods, which always set Target. A null Target indicates the PosView was created via deserialization, reflection, or an internal bug.

Common situations: Debugging output (watch windows, trace logging) that calls ToString() on a Pos that was partially constructed, deserialization creating PosView without Target, or internal bugs during layout where a referenced view was disposed/nullified before ToString is called.

Related errors

• The Viewport of an AdornmentView cannot be modified.
• AdornmentView can only render to their Parent or Parent's Su
• Adornment must be of type Border
• The ShadowStyle of MarginView cannot be set
• The size of an item cannot be negative.

AI-assisted analysis of tui-cs/Terminal.Gui@2e47b11478 (2026-08-13). Data as JSON: /api/errors/93f1aea3b684f8d1. Report an issue: GitHub.