Introduction of th AnyAttribute

README.md

@@ -20,7 +20,7 @@ A Value Object that can be represented by a single scalar.
 
 Technical requirements
 ----------------------
-Because we use .NET standard to support both .NET 4.5 (and higher) as .NET Core (2.0)
+Because we use .NET standard to support both .NET 4.5 (and higher) as .NET Standard (2.0)
 the Visual Studio solution file requires VS2017.3 or higher. Visual Studio can be downloaded
 here: [visualstudio.com/downloads](https://www.visualstudio.com/downloads/).
 
@@ -225,6 +225,7 @@ We're extending the DataAnnotations from Microsoft with some more attributes:
 * [Mandatory] Here the difference with Microsoft's [Required] attribute is that it works for value types as well, it will be invalid if the default value is used.
 * [AllowedValues] and 
 * [ForbiddenValues] make it easy to validate string values, or objects/value types that have a string representation.
+* [Any] Tells that a collection should have at least one item.
 
 Result model
 ------------

src/Qowaiv.ComponentModel/DataAnnotations/AnyAttribute.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections;
+using System.ComponentModel.DataAnnotations;
+
+namespace Qowaiv.ComponentModel.DataAnnotations
+{
+    /// <summary>Specifies that a field should at least have one item in its collection.</summary>
+    public class AnyAttribute : RequiredAttribute
+    {
+        /// <summary>Returns true if the value is not null and the collection
+        /// has any item, otherwise false.
+        /// </summary>
+        public override bool IsValid(object value)
+        {
+            if (value is IEnumerable enumerable)
+            {
+                return enumerable.GetEnumerator().MoveNext();
+            }
+            return base.IsValid(value);
+        }
+    }
+}

test/Qowaiv.ComponentModel.UnitTests/DataAnnotations/AnyAttributeTest.cs

@@ -0,0 +1,38 @@
+using NUnit.Framework;
+using Qowaiv.ComponentModel.DataAnnotations;
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Qowaiv.ComponentModel.UnitTests.DataAnnotations
+{
+    public class AnyAttributeTest
+    {
+        [Test]
+        public void IsValid_Null_IsFalse()
+        {
+            var attribute = new AnyAttribute();
+            IEnumerable collection = null;
+            Assert.IsFalse(attribute.IsValid(collection));
+        }
+
+        [Test]
+        public void IsValid_Empty_IsFalse()
+        {
+            var attribute = new AnyAttribute();
+            var collection = new int[0];
+            Assert.IsFalse(attribute.IsValid(collection));
+        }
+
+        [Test]
+        public void IsValid_1Item_IsTrue()
+        {
+            var attribute = new AnyAttribute();
+            var collection = new int[1];
+            Assert.IsTrue(attribute.IsValid(collection));
+        }
+    }
+}