Redesign AssertionScope and how it travels over chained calls

Build/_build.csproj.DotSettings

@@ -13,6 +13,8 @@
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@@ -21,4 +23,5 @@
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FluentAssertions.sln.DotSettings

@@ -104,6 +104,8 @@
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-{&#xD;
-    // Arrange&#xD;
-    $END$&#xD;
-&#xD;
-    // Act&#xD;
-&#xD;
-&#xD;
-    // Assert&#xD;
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+public void $END$()
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+    // Arrange
+
+
+    // Act
+
+
+    // Assert
 }</s:String>
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Src/FluentAssertions/AndConstraint.cs

@@ -3,15 +3,15 @@
 namespace FluentAssertions;
 
 [DebuggerNonUserCode]
-public class AndConstraint<T>
+public class AndConstraint<TParent>
 {
-    public T And { get; }
+    public TParent And { get; }
 
     /// <summary>
     /// Initializes a new instance of the <see cref="AndConstraint{T}"/> class.
     /// </summary>
-    public AndConstraint(T parentConstraint)
+    public AndConstraint(TParent parent)
     {
-        And = parentConstraint;
+        And = parent;
     }
 }

Src/FluentAssertions/AndWhich.cs

@@ -0,0 +1,70 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using FluentAssertions.Common;
+using FluentAssertions.Execution;
+using FluentAssertions.Formatting;
+
+namespace FluentAssertions;
+
+public class AndWhich<TParent, TSubject> : AndConstraint<TParent>
+{
+    private readonly AssertionChain assertionChain;
+    private readonly string pathPostfix;
+    private readonly Lazy<TSubject> getSubject;
+
+    public AndWhich(TParent parent, TSubject subject, AssertionChain assertionChain, string pathPostfix)
+        : base(parent)
+    {
+        getSubject = new Lazy<TSubject>(() => subject);
+
+        this.assertionChain = assertionChain;
+        this.pathPostfix = pathPostfix;
+    }
+
+    public AndWhich(TParent parent, IEnumerable<TSubject> subjects)
+        : base(parent)
+    {
+        getSubject = new Lazy<TSubject>(() => SingleOrDefault(subjects));
+    }
+
+    /// <summary>
+    /// Returns the single result of a prior assertion that is used to select a nested or collection item.
+    /// </summary>
+    /// <remarks>
+    /// Just a convenience property that returns the same value as <see cref="Which"/>.
+    /// </remarks>
+    public TSubject Subject => Which;
+
+    /// <summary>
+    /// Returns the single result of a prior assertion that is used to select a nested or collection item.
+    /// </summary>
+    public TSubject Which
+    {
+        get
+        {
+            assertionChain.AddCallerPostfix(pathPostfix);
+
+            AssertionChain.ReuseOnce(assertionChain);
+
+            return getSubject.Value;
+        }
+    }
+
+    private static TSubject SingleOrDefault(IEnumerable<TSubject> subjects)
+    {
+        TSubject[] matchedElements = subjects.ToArray();
+        if (matchedElements.Length > 1)
+        {
+            string foundObjects = string.Join(Environment.NewLine,
+                matchedElements.Select(ele => "\t" + Formatter.ToString(ele)));
+
+            string message = "More than one object found.  FluentAssertions cannot determine which object is meant."
+                + $"  Found objects:{Environment.NewLine}{foundObjects}";
+
+            Services.ThrowException(message);
+        }
+
+        return matchedElements.Single();
+    }
+}