[Xaml] cleanup xmlns usage, add XmlnsDefinition (#531)

* [Xaml] cleanup xmlns usage, add XmlnsDefinition

* docs

1 files changed, 25 insertions, 0 deletions

diff --git a/Xamarin.Forms.Core/XmlnsDefinitionAttribute.cs b/Xamarin.Forms.Core/XmlnsDefinitionAttribute.cs
new file mode 100644
index 00000000..2d6b0758
--- /dev/null
+++ b/Xamarin.Forms.Core/XmlnsDefinitionAttribute.cs
@@ -0,0 +1,25 @@
+using System;
+using System.Reflection;
+using System.Diagnostics;
+namespace Xamarin.Forms
+{
+	[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
+	[DebuggerDisplay("{XmlNamespace}, {ClrNamespace}, {AssemblyName}")]
+	public sealed class XmlnsDefinitionAttribute : Attribute
+	{
+		public string XmlNamespace { get; }
+		public string ClrNamespace { get; }
+		public string AssemblyName { get; set; }
+
+		public XmlnsDefinitionAttribute(string xmlNamespace, string clrNamespace)
+		{
+			if (xmlNamespace == null)
+				throw new ArgumentNullException(nameof(xmlNamespace));
+			if (clrNamespace == null)
+				throw new ArgumentNullException(nameof(clrNamespace));
+
+			ClrNamespace = clrNamespace;
+			XmlNamespace = xmlNamespace;
+		}
+	}
+}
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