C#: Add null conditional tests.

Author: michaelnebel

csharp/ql/test/library-tests/nullconditional/NullConditional.cs

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+using System;
+
+public class C
+{
+    public int Field;
+
+    public string Property { get; set; } = "";
+
+    public void M(C c, int[] numbers, bool b)
+    {
+        // Get values.
+        var fieldValue = c?.Field;
+        var propertyValue = c?.Property;
+        var n = numbers?[0];
+
+        // Set values
+        c?.Field = 42;
+        c?.Property = "Hello";
+        numbers?[0] = 7;
+
+        // Set values using operators
+        c?.Field -= 1;
+        c?.Property += " World";
+
+        // Using the return of an assignment
+        var x = c?.Field = 10;
+
+        // Using in a conditional expression
+        int? maybenull = 0;
+        maybenull = (b ? c : null)?.Field;
+    }
+}

csharp/ql/test/library-tests/nullconditional/maybenull.expected

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+| NullConditional.cs:30:22:30:33 | ... ? ... : ... |
+| NullConditional.cs:30:30:30:33 | null |

csharp/ql/test/library-tests/nullconditional/maybenull.ql

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+import csharp
+import semmle.code.csharp.dataflow.Nullness
+
+from MaybeNullExpr mne
+where mne.getFile().getBaseName() = "NullConditional.cs"
+select mne

csharp/ql/test/library-tests/nullconditional/nullconditional.expected

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+| NullConditional.cs:12:26:12:33 | access to field Field |
+| NullConditional.cs:13:29:13:39 | access to property Property |
+| NullConditional.cs:14:17:14:27 | access to array element |
+| NullConditional.cs:17:9:17:16 | access to field Field |
+| NullConditional.cs:18:9:18:19 | access to property Property |
+| NullConditional.cs:19:9:19:19 | access to array element |
+| NullConditional.cs:22:9:22:16 | access to field Field |
+| NullConditional.cs:22:9:22:16 | access to field Field |
+| NullConditional.cs:23:9:23:19 | access to property Property |
+| NullConditional.cs:23:9:23:19 | access to property Property |
+| NullConditional.cs:26:17:26:24 | access to field Field |
+| NullConditional.cs:30:21:30:41 | access to field Field |

csharp/ql/test/library-tests/nullconditional/nullconditional.ql

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+import csharp
+
+from QualifiableExpr e
+where e.isConditional()
+select e

csharp/ql/test/library-tests/nullconditional/options

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+semmle-extractor-options: /nostdlib /noconfig
+semmle-extractor-options: --load-sources-from-project:${testdir}/../../resources/stubs/_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj