More unit tests

Also ensured the DoubleToCornerRadiusConverter returns a CornerRadius of 0 for negative values.

MaterialDesignThemes.UITests/WPF/Cards/OutlinedCardTests.cs

@@ -27,4 +27,26 @@ public async Task OutlinedCard_UsesThemeColorForBorder()
 
         recorder.Success();
     }
-}
+
+    [Fact]
+    public async Task OutlinedCard_UniformCornerRadiusApplied_AppliesCornerRadiusOnBorder()
+    {
+        await using var recorder = new TestRecorder(App);
+
+        //Arrange
+        IVisualElement<Card> card = await LoadXaml<Card>(
+            @"<materialDesign:Card Content=""Hello World"" Style=""{StaticResource MaterialDesignOutlinedCard}"" UniformCornerRadius=""5"" />");
+        IVisualElement<Border> internalBorder = await card.GetElement<Border>();
+
+        //Act
+        CornerRadius? internalBorderCornerRadius = await internalBorder.GetCornerRadius();
+
+        //Assert
+        Assert.Equal(5, internalBorderCornerRadius.Value.TopLeft);
+        Assert.Equal(5, internalBorderCornerRadius.Value.TopRight);
+        Assert.Equal(5, internalBorderCornerRadius.Value.BottomRight);
+        Assert.Equal(5, internalBorderCornerRadius.Value.BottomLeft);
+
+        recorder.Success();
+    }
+}

MaterialDesignThemes.Wpf.Tests/Converters/DoubleToCornerRadiusConverterTests.cs

@@ -0,0 +1,26 @@
+using System.Globalization;
+using MaterialDesignThemes.Wpf.Converters;
+using Xunit;
+
+namespace MaterialDesignThemes.Wpf.Tests.Converters;
+
+public class DoubleToCornerRadiusConverterTests
+{
+    [Theory]
+    [InlineData(-0.16, 0.0)]
+    [InlineData(0.16, 0.16)]
+    [InlineData(5.0, 5.0)]
+    public void AllCultureParseParameterCorrectly(object parameter, double expectedCornerRadius)
+    {
+        var converter = new DoubleToCornerRadiusConverter();
+        foreach (var culture in CultureInfo.GetCultures(CultureTypes.AllCultures))
+        {
+            var cornerRadius = (CornerRadius?)converter.Convert(parameter, typeof(CornerRadius), parameter, culture);
+
+            Assert.Equal(expectedCornerRadius, cornerRadius.Value.TopLeft);
+            Assert.Equal(expectedCornerRadius, cornerRadius.Value.TopRight);
+            Assert.Equal(expectedCornerRadius, cornerRadius.Value.BottomRight);
+            Assert.Equal(expectedCornerRadius, cornerRadius.Value.BottomLeft);
+        }
+    }
+}

MaterialDesignThemes.Wpf/Converters/DoubleToCornerRadiusConverter.cs

@@ -7,7 +7,7 @@ internal class DoubleToCornerRadiusConverter : IValueConverter
 {
     public static readonly DoubleToCornerRadiusConverter Instance = new();
 
-    public object Convert(object value, Type targetType, object parameter, CultureInfo culture) => new CornerRadius((double)value);
+    public object Convert(object value, Type targetType, object parameter, CultureInfo culture) => new CornerRadius(Math.Max(0, (double)value));
 
     public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) => throw new NotImplementedException();
 }