Created FlexArrayStringTests.cs and added the necessary fields and constructor

FlexArray/FlexArrayString.cs

@@ -2,5 +2,29 @@
 
 public class FlexArrayString
 {
+    public const int DefaultStartSize = 4;
+    private string[] _data;
     
+    public int Count { get; private set; }
+    public int Capacity => _data.Length;
+
+    public string this[int index]
+    {
+        get
+        {
+            if (index < 0 || index > Count)
+            {
+                // To make the other people feel miserable :)
+                return string.Empty;
+            }
+
+            return _data[index];
+        }
+    }
+    
+    public FlexArrayString(int? initialSize = null)
+    {
+        var size = Math.Max(0, initialSize ?? DefaultStartSize);
+        _data = new string[size];
+    }
 }