[C] avoid Enum.TryParse for resolving NamedSizes (#4501)

Enum.TryParse is quite expensive. replace that by simple value check for
known values. Keep the TryParse as a safeguard in case we ever add new
values to it.

Xamarin.Forms.Core/FontSizeConverter.cs

@@ -4,30 +4,40 @@ using Xamarin.Forms.Xaml;
 
 namespace Xamarin.Forms
 {
-	[Xaml.TypeConversion(typeof(double))]
+	[TypeConversion(typeof(double))]
 	public class FontSizeConverter : TypeConverter, IExtendedTypeConverter
 	{
 		[Obsolete("IExtendedTypeConverter.ConvertFrom is obsolete as of version 2.2.0. Please use ConvertFromInvariantString (string, IServiceProvider) instead.")]
 		object IExtendedTypeConverter.ConvertFrom(CultureInfo culture, object value, IServiceProvider serviceProvider)
-		{
-			return ((IExtendedTypeConverter)this).ConvertFromInvariantString(value as string, serviceProvider);
-		}
+			=> ((IExtendedTypeConverter)this).ConvertFromInvariantString(value as string, serviceProvider);
 
 		object IExtendedTypeConverter.ConvertFromInvariantString(string value, IServiceProvider serviceProvider)
 		{
-			if (value != null)
-			{
-				double size;
-				if (double.TryParse(value, NumberStyles.Number, CultureInfo.InvariantCulture, out size))
+			if (value != null) {
+				value = value.Trim();
+				if (double.TryParse(value, NumberStyles.Number, CultureInfo.InvariantCulture, out double size))
 					return size;
+
 				var ignoreCase = (serviceProvider?.GetService(typeof(IConverterOptions)) as IConverterOptions)?.IgnoreCase ?? false;
-				NamedSize namedSize;
-				if (Enum.TryParse(value, ignoreCase, out namedSize))
-				{
-					Type type;
-					var valueTargetProvider = serviceProvider.GetService(typeof(IProvideValueTarget)) as IProvideValueTarget;
-					type = valueTargetProvider != null ? valueTargetProvider.TargetObject.GetType() : typeof(Label);
-					return Device.GetNamedSize(namedSize, type, false);
+				var sc = ignoreCase ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal;
+				NamedSize? namedSize = null;
+
+				if (value.Equals(nameof(NamedSize.Default), sc))
+					namedSize = NamedSize.Default;
+				else if (value.Equals(nameof(NamedSize.Micro), sc))
+					namedSize = NamedSize.Micro;
+				else if (value.Equals(nameof(NamedSize.Small), sc))
+					namedSize = NamedSize.Small;
+				else if (value.Equals(nameof(NamedSize.Medium), sc))
+					namedSize = NamedSize.Medium;
+				else if (value.Equals(nameof(NamedSize.Large), sc))
+					namedSize = NamedSize.Large;
+				else if (Enum.TryParse(value, ignoreCase, out NamedSize ns))
+					namedSize = ns;
+
+				if (namedSize.HasValue) {
+					var type = serviceProvider.GetService(typeof(IProvideValueTarget)) is IProvideValueTarget valueTargetProvider ? valueTargetProvider.TargetObject.GetType() : typeof(Label);
+					return Device.GetNamedSize(namedSize.Value, type, false);
 				}
 			}
 			throw new InvalidOperationException(string.Format("Cannot convert \"{0}\" into {1}", value, typeof(double)));
@@ -35,13 +45,22 @@ namespace Xamarin.Forms
 
 		public override object ConvertFromInvariantString(string value)
 		{
-			if (value != null)
-			{
-				double size;
-				if (double.TryParse(value, NumberStyles.Number, CultureInfo.InvariantCulture, out size))
+			if (value != null) {
+				if (double.TryParse(value, NumberStyles.Number, CultureInfo.InvariantCulture, out double size))
 					return size;
-				NamedSize namedSize;
-				if (Enum.TryParse(value, out namedSize))
+				value = value.Trim();
+
+				if (value.Equals(nameof(NamedSize.Default), StringComparison.Ordinal))
+					return Device.GetNamedSize(NamedSize.Default, typeof(Label), false);
+				if (value.Equals(nameof(NamedSize.Micro), StringComparison.Ordinal))
+					return Device.GetNamedSize(NamedSize.Micro, typeof(Label), false);
+				if (value.Equals(nameof(NamedSize.Small), StringComparison.Ordinal))
+					return Device.GetNamedSize(NamedSize.Small, typeof(Label), false);
+				if (value.Equals(nameof(NamedSize.Medium), StringComparison.Ordinal))
+					return Device.GetNamedSize(NamedSize.Medium, typeof(Label), false);
+				if (value.Equals(nameof(NamedSize.Large), StringComparison.Ordinal))
+					return Device.GetNamedSize(NamedSize.Large, typeof(Label), false);
+				if (Enum.TryParse(value, out NamedSize namedSize))
 					return Device.GetNamedSize(namedSize, typeof(Label), false);
 			}
 			throw new InvalidOperationException(string.Format("Cannot convert \"{0}\" into {1}", value, typeof(double)));