Added very nice DataGrid control

UserService.DatabaseLayer/Queryable.cs

@@ -0,0 +1,17 @@
+using System.Linq.Expressions;
+
+// ReSharper disable once CheckNamespace
+namespace System.Linq
+{
+    public static class Queryable
+    {
+        /// <summary>
+        /// Makes a where filtering, if it is not null.
+        /// </summary>
+        public static IQueryable<TSource> WhereOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>>? predicate)
+        {
+            if (source == null) throw new ArgumentNullException(nameof(source));
+            return predicate is null ? source : source.Where(predicate);
+        }
+    }
+}