tui-cs/Terminal.Gui · error · ArgumentNullException

X cannot be null

Error message

X cannot be null

What it means

The X property (horizontal position) must be a non-null Pos object. Terminal.Gui's layout system is fully declarative — even 'no position' is expressed as Pos.Absolute(0), never as null. A null Pos would leave the layout engine unable to compute a coordinate.

Source

Thrown at Terminal.Gui/ViewBase/View.Layout.cs:288

    ///         resulting in the
    ///         view being laid out and redrawn as appropriate in the next iteration.
    ///     </para>
    ///     <para>
    ///         Changing this property will cause <see cref="Frame"/> to be updated.
    ///     </para>
    ///     <para>The default value is <c>Pos.Absolute (0)</c>.</para>
    /// </remarks>
    public Pos X
    {
        get => _x;
        set
        {
            if (Equals (_x, value))
            {
                return;
            }

            _x = value ?? throw new ArgumentNullException (nameof (value), @$"{nameof (X)} cannot be null");

            PosDimSet ();

            NeedsClearScreenNextIteration ();
        }
    }

    private Pos _y = Pos.Absolute (0);

    /// <summary>
    ///     Gets or sets the declarative vertical position for the view.
    /// </summary>
    /// <value>The <see cref="Pos"/> object representing the Y position.</value>
    /// <remarks>
    ///     <para>
    ///         See the View Layout Deep Dive for more information:
    ///         <see href="https://tui-cs.github.io/Terminal.Gui/docs/layout.html"/>
    ///     </para>

View on GitHub (pinned to 2e47b11478)

Solutions

1. Use Pos.Absolute(n) for fixed positions.
2. Use Pos.Center(), Pos.Percent(n), or Pos.Left(otherView) for relative layout.
3. If conditionally clearing, set Pos.Absolute(0) rather than null.

Example fix

// before
view.X = maybePos;
// after
view.X = maybePos ?? Pos.Absolute(0);

view.X = pos ?? throw new ArgumentNullException(nameof(pos));
// or: view.X = pos ?? Pos.Absolute(0);

static bool IsValidPos (Pos? p) => p is not null;

When it happens

Trigger: Assigning view.X = null; or a Pos-typed variable that is null (e.g. a field never initialized, or a result of a method that returned null).

Common situations: Nullable Pos returned from a helper that returns null on edge cases; conditional initialization that leaves X unset; deserialization gap.

Related errors

• Y cannot be null
• The size of an item cannot be negative.
• Target
• Content width cannot be negative.
• Content height cannot be negative.

AI-assisted analysis of tui-cs/Terminal.Gui@2e47b11478 (2026-08-13). Data as JSON: /api/errors/d6ab601b2d2b5448. Report an issue: GitHub.