fix: Address CA1847 rule

src/Avalonia.Base/AvaloniaProperty.cs

@@ -41,7 +41,7 @@ namespace Avalonia
         {
             _ = name ?? throw new ArgumentNullException(nameof(name));
 
-            if (name.Contains("."))
+            if (name.Contains('.'))
             {
                 throw new ArgumentException("'name' may not contain periods.");
             }

src/Avalonia.Base/AvaloniaPropertyRegistry.cs

@@ -228,7 +228,7 @@ namespace Avalonia
             _ = type ?? throw new ArgumentNullException(nameof(type));
             _ = name ?? throw new ArgumentNullException(nameof(name));
 
-            if (name.Contains("."))
+            if (name.Contains('.'))
             {
                 throw new InvalidOperationException("Attached properties not supported.");
             }

src/Avalonia.Base/Compatibility/StringCompatibilityExtensions.cs

@@ -0,0 +1,12 @@
+using System.Runtime.CompilerServices;
+
+namespace System;
+
+#if !NET6_0_OR_GREATER
+public static class StringCompatibilityExtensions
+{
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static bool Contains(this string str, char search) =>
+        str.Contains(search.ToString());
+}
+#endif

src/Avalonia.Base/Media/UnicodeRange.cs

@@ -163,7 +163,7 @@ namespace Avalonia.Media
                             throw new FormatException("Could not parse specified Unicode range segment.");
                         }
 
-                        if (!single.Value.Contains("?"))
+                        if (!single.Value.Contains('?'))
                         {
                             start = int.Parse(single.Groups[1].Value, System.Globalization.NumberStyles.HexNumber);
                             end = start;

src/Avalonia.Controls/Flyouts/FlyoutBase.cs

@@ -597,7 +597,7 @@ namespace Avalonia.Controls.Primitives
             for (int i = presenter.Classes.Count - 1; i >= 0; i--)
             {
                 if (!classes.Contains(presenter.Classes[i]) &&
-                    !presenter.Classes[i].Contains(":"))
+                    !presenter.Classes[i].Contains(':'))
                 {
                     presenter.Classes.RemoveAt(i);
                 }

src/Avalonia.Controls/NumericUpDown/NumericUpDown.cs

@@ -1151,8 +1151,8 @@ namespace Avalonia.Controls
             if (PIndex >= 0)
             {
                 //stringToTest contains a "P" between 2 "'", it's considered as text, not percent
-                var isText = stringToTest.Substring(0, PIndex).Contains("'")
+                var isText = stringToTest.Substring(0, PIndex).Contains('\'')
-                             && stringToTest.Substring(PIndex, FormatString.Length - PIndex).Contains("'");
+                             && stringToTest.Substring(PIndex, FormatString.Length - PIndex).Contains('\'');
 
                 return !isText;
             }

src/Avalonia.Remote.Protocol/MetsysBson.cs

@@ -1364,13 +1364,13 @@ namespace Metsys.Bson
             var optionsString = ReadName();
 
             var options = RegexOptions.None;
-            if (optionsString.Contains("e")) options = options | RegexOptions.ECMAScript;
+            if (optionsString.Contains('e')) options = options | RegexOptions.ECMAScript;
-            if (optionsString.Contains("i")) options = options | RegexOptions.IgnoreCase;
+            if (optionsString.Contains('i')) options = options | RegexOptions.IgnoreCase;
-            if (optionsString.Contains("l")) options = options | RegexOptions.CultureInvariant;
+            if (optionsString.Contains('l')) options = options | RegexOptions.CultureInvariant;
-            if (optionsString.Contains("m")) options = options | RegexOptions.Multiline;
+            if (optionsString.Contains('m')) options = options | RegexOptions.Multiline;
-            if (optionsString.Contains("s")) options = options | RegexOptions.Singleline;
+            if (optionsString.Contains('s')) options = options | RegexOptions.Singleline;
-            if (optionsString.Contains("w")) options = options | RegexOptions.IgnorePatternWhitespace;
+            if (optionsString.Contains('w')) options = options | RegexOptions.IgnorePatternWhitespace;
-            if (optionsString.Contains("x")) options = options | RegexOptions.ExplicitCapture;
+            if (optionsString.Contains('x')) options = options | RegexOptions.ExplicitCapture;
 
             return new Regex(pattern, options);
         }

src/Markup/Avalonia.Markup.Xaml/Converters/TimeSpanTypeConverter.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Markup.Xaml.Converters
         public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
         {
             var valueStr = (string)value;
-            if (!valueStr.Contains(":"))
+            if (!valueStr.Contains(':'))
             {
                 // shorthand seconds format (ie. "0.25")
                 var secs = double.Parse(valueStr, CultureInfo.InvariantCulture);
@@ -25,4 +25,4 @@ namespace Avalonia.Markup.Xaml.Converters
             return base.ConvertFrom(context, culture, value);
         }
     }
-}
+}