Add test for new LinearExpr.SolutionValue() method

ortools/linear_solver/csharp/LinearSolverTests.cs

@@ -442,5 +442,28 @@ static void testSetHintAndSolverGetters()
 
         solver.SetHint(new Variable[] { x, y }, new double[] { 2.0, 3.0 });
     }
+
+    [Fact]
+    static void Given_a_LinearExpr_and_a_solution_When_SolutionValue_is_called_then_the_result_is_correct()
+    {
+        Console.WriteLine(nameof(Given_a_LinearExpr_and_a_solution_When_SolutionValue_is_called_then_the_result_is_correct));
+        Solver solver = Solver.CreateSolver("glop");
+        // x, y and z are fixed; we don't want to test the solver here.
+        Variable x = solver.MakeIntVar(3, 3, "x");
+        Variable y = solver.MakeIntVar(4, 4, "y");
+        Variable z = solver.MakeIntVar(5, 5, "z");
+
+        LinearExpr part1 = x * 2;         // 6
+        LinearExpr part2 = y * 3 + z + 4; // 21
+        LinearExpr objective = part1 + part2;   // 27
+        LinearExpr anew = new();
+        solver.Maximize(objective);
+        solver.Solve();
+        Assert.Equal(27, objective.SolutionValue(), precision: 9);
+        Assert.Equal(6, part1.SolutionValue(), precision: 9);
+        Assert.Equal(21, part2.SolutionValue(), precision: 9);
+        Assert.Equal(0, anew.SolutionValue(), precision: 9);
+        Assert.Equal(27, (objective + anew).SolutionValue(), precision: 9);
+    }
 }
 } // namespace Google.OrTools.Tests