Fix NRE in "Literal.Text" when the constant value is null.

src/Compilers/CSharp/Test/Semantic/Diagnostics/OperationAnalyzerTests.cs

@@ -1560,5 +1560,43 @@ public void M0(C p)
                 Diagnostic(ConditionalAccessOperationTestAnalyzer.ConditionalAccessOperationDescriptor.Id, "Field1?.M0(null)").WithLocation(32, 9),
                 Diagnostic(ConditionalAccessOperationTestAnalyzer.ConditionalAccessInstanceOperationDescriptor.Id, "Field1").WithLocation(32, 9));
         }
+
+        [WorkItem(9116, "https://github.com/dotnet/roslyn/issues/9116")]
+        [Fact]
+        public void LiteralCSharp()
+        {
+            const string source = @"
+public class A
+{
+    public void M()
+    {
+        object a = null;
+    }
+}
+
+struct S
+{
+    void M(
+        int i = 1,
+        string str = ""hello"",
+        object o = null,
+        S s = default(S))
+    {
+        M();
+    }
+}
+";
+            CreateCompilationWithMscorlib45(source)
+             .VerifyDiagnostics(Diagnostic(ErrorCode.WRN_UnreferencedVarAssg, "a").WithArguments("a").WithLocation(6, 16))
+             .VerifyAnalyzerDiagnostics(new DiagnosticAnalyzer[] { new LiteralTestAnalyzer() }, null, null, false,
+                Diagnostic("Literal", "null").WithArguments("null").WithLocation(6, 20),
+                Diagnostic("Literal", "1").WithArguments("1").WithLocation(13, 17),
+                Diagnostic("Literal", @"""hello""").WithArguments(@"""hello""").WithLocation(14, 22),
+                Diagnostic("Literal", "null").WithArguments("null").WithLocation(15, 20),
+                Diagnostic("Literal", "M()").WithArguments("1").WithLocation(18, 9),
+                Diagnostic("Literal", "M()").WithArguments(@"""hello""").WithLocation(18, 9),
+                Diagnostic("Literal", "M()").WithArguments("null").WithLocation(18, 9),
+                Diagnostic("Literal", "M()").WithArguments("null").WithLocation(18, 9));
+        }
     }
 }
\ No newline at end of file

src/Compilers/Core/CodeAnalysisTest/Diagnostics/OperationTestAnalyzer.cs

@@ -1773,4 +1773,33 @@ public sealed override void Initialize(AnalysisContext context)
                  OperationKind.LoopStatement);
         }
     }
+
+    public class LiteralTestAnalyzer : DiagnosticAnalyzer
+    {
+        private const string ReliabilityCategory = "Reliability";
+        
+        public static readonly DiagnosticDescriptor LiteralDescriptor = new DiagnosticDescriptor(
+            "Literal",
+            "A literal is found",
+            "A literal of value {0} is found",
+            ReliabilityCategory,
+            DiagnosticSeverity.Warning,
+            isEnabledByDefault: true);
+
+        public sealed override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics
+        {
+            get { return ImmutableArray.Create(LiteralDescriptor); }
+        }
+
+        public sealed override void Initialize(AnalysisContext context)
+        {
+            context.RegisterOperationAction(
+                 (operationContext) =>
+                 {
+                     var literal = (ILiteralExpression)operationContext.Operation;
+                     operationContext.ReportDiagnostic(Diagnostic.Create(LiteralDescriptor, literal.Syntax.GetLocation(), literal.Text));
+                 },
+                 OperationKind.LiteralExpression);
+        }
+    }
 }
\ No newline at end of file

src/Compilers/Core/Portable/Compilation/Expression.cs

@@ -308,7 +308,7 @@ public Literal(ConstantValue value, ITypeSymbol resultType, SyntaxNode syntax)
             this.Syntax = syntax;
         }
 
-        public string Text => _value.Value.ToString();
+        public string Text => _value.GetValueToDisplay();
 
         public ITypeSymbol Type { get; }
 

src/Compilers/Core/Portable/Compilation/OperationWalker.cs

@@ -193,7 +193,7 @@ public override void VisitEndStatement(IEndStatement operation)
         public override void VisitInvocationExpression(IInvocationExpression operation)
         {
             Visit(operation.Instance);
-            VisitArray(operation.ArgumentsInSourceOrder);
+            VisitArray(operation.ArgumentsInParameterOrder);
         }
 
         public override void VisitArgument(IArgument operation)