Добавил новые интерфейсы

AMessanger.Core/Result.cs

@@ -0,0 +1,21 @@
+namespace AMessanger.Core;
+
+public class Result<T>
+{
+    public T? Value { get; }
+
+    public string? ErrorMessage { get; }
+
+    public bool IsSuccess { get; }
+
+    private Result(T? value, bool isSuccess, string? message)
+    {
+        Value = value;
+        IsSuccess = isSuccess;
+        ErrorMessage = message;
+    }
+
+    public static Result<T> Error(string message) => new(default, false, message);
+
+    public static Result<T> Success(T value) => new(value, true, null);
+}