[Image Resizer] Upgraded to .NET Core 3.1 (#7317)

* project converted to sdk style

* image resizer core31

* image resizer test core31

* project and setup fixes

src/modules/imageresizer/ui/Helpers/Observable.cs

@@ -0,0 +1,27 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+
+namespace ImageResizer.Helpers
+{
+    public class Observable : INotifyPropertyChanged
+    {
+        public event PropertyChangedEventHandler PropertyChanged;
+
+        protected void Set<T>(ref T storage, T value, [CallerMemberName] string propertyName = null)
+        {
+            if (Equals(storage, value))
+            {
+                return;
+            }
+
+            storage = value;
+            OnPropertyChanged(propertyName);
+        }
+
+        protected void OnPropertyChanged(string propertyName) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+    }
+}

src/modules/imageresizer/ui/Helpers/RelayCommand.cs

@@ -0,0 +1,61 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Windows.Input;
+
+namespace ImageResizer.Helpers
+{
+    public class RelayCommand : ICommand
+    {
+        private readonly Action _execute;
+        private readonly Func<bool> _canExecute;
+
+        public event EventHandler CanExecuteChanged;
+
+        public RelayCommand(Action execute)
+            : this(execute, null)
+        {
+        }
+
+        public RelayCommand(Action execute, Func<bool> canExecute)
+        {
+            _execute = execute ?? throw new ArgumentNullException(nameof(execute));
+            _canExecute = canExecute;
+        }
+
+        public bool CanExecute(object parameter) => _canExecute == null || _canExecute();
+
+        public void Execute(object parameter) => _execute();
+
+        public void OnCanExecuteChanged() => CanExecuteChanged?.Invoke(this, EventArgs.Empty);
+    }
+
+    [System.Diagnostics.CodeAnalysis.SuppressMessage("StyleCop.CSharp.MaintainabilityRules", "SA1402:File may only contain a single type", Justification = "abstract T and abstract")]
+    public class RelayCommand<T> : ICommand
+    {
+        private readonly Action<T> execute;
+
+        private readonly Func<T, bool> canExecute;
+
+        public event EventHandler CanExecuteChanged;
+
+        public RelayCommand(Action<T> execute)
+            : this(execute, null)
+        {
+        }
+
+        public RelayCommand(Action<T> execute, Func<T, bool> canExecute)
+        {
+            this.execute = execute ?? throw new ArgumentNullException(nameof(execute));
+            this.canExecute = canExecute;
+        }
+
+        public bool CanExecute(object parameter) => canExecute == null || canExecute((T)parameter);
+
+        public void Execute(object parameter) => execute((T)parameter);
+
+        public void OnCanExecuteChanged() => CanExecuteChanged?.Invoke(this, EventArgs.Empty);
+    }
+}