Fixed a crash when passing null as an std::string on Unix. (#1012)

Fixed a crash when passing null as an std::string on Unix

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -2743,6 +2743,15 @@ namespace CppSharp.Generators.CSharp
                 }
             }
 
+            // special validation when constructing std::string as it cannot take null as a value
+            if (method != null && method.OriginalName == "basic_string" &&
+                method.TranslationUnit.IsSystemHeader)
+            {
+                WriteLine($"if (ReferenceEquals({method.Parameters[0].Name}, null))");
+                WriteLineIndent($@"throw new global::System.ArgumentNullException({
+                    method.Parameters[0].Name}, ""The underlying std::string cannot take null."");");
+            }
+
             if (needsReturn && !originalFunction.HasIndirectReturnTypeParameter)
                 Write("var {0} = ", Helpers.ReturnIdentifier);
 

tests/Common/Common.Tests.cs

@@ -765,6 +765,15 @@ This is a very long string. This is a very long string. This is a very long stri
         }
     }
 
+    [Test, Platform(Exclude = "Win")]
+    public void TestNullStdString()
+    {
+        using (var hasStdString = new HasStdString())
+        {
+            Assert.That(() => hasStdString.TestStdString(null), Throws.ArgumentNullException);
+        }
+    }
+
     private class CustomDerivedFromVirtual : AbstractWithVirtualDtor
     {
         public override void Abstract()