override Description in constraints, might be enough to support NUnit4

src/main/net-constraints-nunit3/CompareConstraint.cs

@@ -60,6 +60,10 @@ public static CompareConstraint IsSimilarTo(object control) {
             return new CompareConstraint(control).CheckForSimilar();
         }
 
+        // for compatibility with NUnit 4.x
+        /// <inheritdoc/>
+        public override string Description { get; protected set; }
+
         private CompareConstraint CheckForSimilar() {
             diffBuilder.CheckForSimilar();
             checkFor = ComparisonResult.SIMILAR;

src/main/net-constraints-nunit3/EvaluateXPathConstraint.cs

@@ -105,6 +105,10 @@ public static EvaluateXPathConstraint HasXPath(string xPath, IConstraint valueCo
             return new EvaluateXPathConstraint(xPath, valueConstraint);
         }
 
+        // for compatibility with NUnit 4.x
+        /// <inheritdoc/>
+        public override string Description { get; protected set; }
+
         /// <inheritdoc/>
         public override ConstraintResult ApplyTo<TActual>(TActual actual)
         {

src/main/net-constraints-nunit3/HasXPathConstraint.cs

@@ -80,6 +80,10 @@ public HasXPathConstraint(string xPath)
             this.xPath = xPath;
         }
 
+        // for compatibility with NUnit 4.x
+        /// <inheritdoc/>
+        public override string Description { get; protected set; }
+
         /// <inheritdoc/>
         public override ConstraintResult ApplyTo<TActual>(TActual actual)
         {

src/main/net-constraints-nunit3/ValidationConstraints.cs

@@ -62,6 +62,10 @@ public SchemaValidConstraint(XmlSchema schema) : base(schema) {
             validator.Schema = schema;
         }
 
+        // for compatibility with NUnit 4.x
+        /// <inheritdoc/>
+        public override string Description { get; protected set; }
+
         /// <inheritdoc/>
         public override ConstraintResult ApplyTo<TActual>(TActual actual) {
             result = validator.ValidateInstance(actual as ISource);