C#: Add more test examples for cs/missing-readonly-modifier.

Author: michaelnebel

csharp/ql/test/query-tests/Language Abuse/MissedReadonlyOpportunity/MissedReadonlyOpportunity.cs

@@ -2,22 +2,26 @@ class MissedReadonlyOpportunity<T>
 {
     public int Bad1; // $ Alert
     public T Bad2; // $ Alert
+    public Immutable Bad3; // $ Alert
     public readonly int Good1;
     public readonly int Good2 = 0;
     public const int Good3 = 0;
     public int Good4;
     public readonly T Good5;
     public T Good6;
+    public Mutable Good7;
 
     public MissedReadonlyOpportunity(int i, T t)
     {
         Bad1 = i;
         Bad2 = t;
+        Bad3 = new Immutable();
         Good1 = i;
         Good2 = i;
         Good4 = i;
         Good5 = t;
         Good6 = t;
+        Good7 = new Mutable();
     }
 
     public void M(int i)
@@ -27,3 +31,54 @@ public void M(int i)
         x.Good6 = false;
     }
 }
+
+struct Mutable
+{
+    private int x;
+    public int Mutate()
+    {
+        x = x + 1;
+        return x;
+    }
+}
+
+readonly struct Immutable { }
+
+class Tree
+{
+    private Tree? Parent;
+    private Tree? Left; // $ Alert
+    private readonly Tree? Right;
+
+    public Tree(Tree left, Tree right)
+    {
+        this.Left = left;
+        this.Right = right;
+        left.Parent = this;
+        right.Parent = this;
+    }
+
+    public Tree()
+    {
+        Left = null;
+        Right = null;
+    }
+}
+
+class StaticFields
+{
+    static int X; // $ Alert
+    static int Y;
+
+    // Static constructor
+    static StaticFields()
+    {
+        X = 0;
+    }
+
+    // Instance constructor
+    public StaticFields(int y)
+    {
+        Y = y;
+    }
+}

csharp/ql/test/query-tests/Language Abuse/MissedReadonlyOpportunity/MissedReadonlyOpportunity.expected

@@ -1,3 +1,14 @@
+#select
 | MissedReadonlyOpportunity.cs:3:16:3:19 | Bad1 | Field 'Bad1' can be 'readonly'. |
 | MissedReadonlyOpportunity.cs:4:14:4:17 | Bad2 | Field 'Bad2' can be 'readonly'. |
+| MissedReadonlyOpportunity.cs:5:22:5:25 | Bad3 | Field 'Bad3' can be 'readonly'. |
+| MissedReadonlyOpportunity.cs:12:20:12:24 | Good7 | Field 'Good7' can be 'readonly'. |
+| MissedReadonlyOpportunity.cs:49:19:49:24 | Parent | Field 'Parent' can be 'readonly'. |
+| MissedReadonlyOpportunity.cs:50:19:50:22 | Left | Field 'Left' can be 'readonly'. |
+| MissedReadonlyOpportunity.cs:70:16:70:16 | X | Field 'X' can be 'readonly'. |
+| MissedReadonlyOpportunity.cs:71:16:71:16 | Y | Field 'Y' can be 'readonly'. |
 | MissedReadonlyOpportunityBad.cs:3:9:3:13 | Field | Field 'Field' can be 'readonly'. |
+testFailures
+| MissedReadonlyOpportunity.cs:12:20:12:24 | Field 'Good7' can be 'readonly'. | Unexpected result: Alert |
+| MissedReadonlyOpportunity.cs:49:19:49:24 | Field 'Parent' can be 'readonly'. | Unexpected result: Alert |
+| MissedReadonlyOpportunity.cs:71:16:71:16 | Field 'Y' can be 'readonly'. | Unexpected result: Alert |