AvaloniaUI/Avalonia · error · ArgumentException

Invalid KeySpline X1 value. Must be >= 0.0 and <= 1.0.

Error message

Invalid KeySpline X1 value. Must be >= 0.0 and <= 1.0.

What it means

Thrown by the ControlPointX1 setter when the supplied X1 value fails IsValidXValue, i.e. it is less than 0.0 or greater than 1.0. The constraint applies only to X coordinates because they represent normalized time on the bezier easing curve; Y coordinates (progress) are unconstrained.

Source

Thrown at src/Avalonia.Base/Animation/KeySpline.cs:105

            return new KeySpline(tokenizer.ReadDouble(), tokenizer.ReadDouble(), tokenizer.ReadDouble(), tokenizer.ReadDouble());
        }

        /// <summary>
        /// X coordinate of the first control point
        /// </summary>
        public double ControlPointX1
        {
            get => _controlPointX1;
            set
            {
                if (IsValidXValue(value))
                {
                    _controlPointX1 = value;
                    _isDirty = true;
                }
                else
                {
                    throw new ArgumentException("Invalid KeySpline X1 value. Must be >= 0.0 and <= 1.0.");
                }
            }
        }

        /// <summary>
        /// Y coordinate of the first control point
        /// </summary>
        public double ControlPointY1
        {
            get => _controlPointY1;
            set
            {
                _controlPointY1 = value;
                _isDirty = true;
            }
        }

        /// <summary>

View on GitHub (pinned to 11c5427268)

Solutions

1. Clamp the value to [0,1] before assigning: Math.Clamp(v, 0.0, 1.0).
2. Use the constructor KeySpline(x1,y1,x2,y2) only after validating, or set Y coordinates freely and keep X in range.
3. If the value comes from a string, validate with double.TryParse under InvariantCulture before assignment.

Example fix

// before
spline.ControlPointX1 = 1.2;

// after
spline.ControlPointX1 = Math.Clamp(1.2, 0.0, 1.0);

double x1 = Math.Clamp(rawX1, 0.0, 1.0);
spline.ControlPointX1 = x1;

static bool IsValidX(double v) => double.IsFinite(v) && v >= 0.0 && v <= 1.0;

try { spline.ControlPointX1 = v; }
catch (ArgumentException) { spline.ControlPointX1 = Math.Clamp(v, 0.0, 1.0); }

When it happens

Trigger: Assigning a number outside [0,1] to KeySpline.ControlPointX1 directly, or via reflection/serialization; parsing a malformed string into a KeySpline and then mutating X1.

Common situations: Treating the first control point like a free 2D point and setting X to values like 2.0 or -0.5; culture-related parse bugs producing NaN that then get assigned.

Related errors

• {nameof(KeySpline)} must have X coordinates >= 0.0 and <= 1.
• Invalid KeySpline X2 value. Must be >= 0.0 and <= 1.0.
• Transition elements have different parents.
• No Setter property assigned.
• Unknown PlaybackBehavior value: {_playbackBehavior}

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/4aaf89b41f64cc11. Report an issue: GitHub.