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Filters/XmlAttributeFilter.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+using System.Windows.Markup;
+using System.Xml;
+
+namespace example_csharp_app.Filters;
+
+[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter, AllowMultiple = false)]
+public class XmlAttribute : ValidationAttribute
+{
+
+    public XmlAttribute(): base("Passed string is not XML")
+    {}
+
+    /// <summary>
+    ///     Override of <see cref="ValidationAttribute.IsValid(object)" />
+    /// </summary>
+    /// <param name="value">The value to test</param>
+    /// <returns>
+    ///     Returns <see langword="false" /> if the <paramref name="value" /> is null or an empty string.
+    ///     If <see cref="AllowEmptyStrings" /> then <see langword="true" /> is returned for empty strings.
+    /// </returns>
+    public override bool IsValid(object? value)
+    {
+        if (value is null || value is not string)
+        {
+            return false;
+        }
+
+        string possiblyXml = ((string)value).Trim();
+        if (!possiblyXml.StartsWith('<') && !possiblyXml.EndsWith('>')) 
+        {
+            return false;
+        }
+        XmlDocument xml = new XmlDocument();
+        try
+        {
+            xml.LoadXml(possiblyXml);
+        }
+        catch (XmlException e)
+        {
+            // logme?
+            return false;
+        }
+
+        return true;
+    }
+    
+}