Write correct C# for unsafe methods

- #4488 problem (4)
- avoid C# compilation errors
- generate `unsafe` keyword when base constructor call is unsafe
- generate `unsafe` keyword for unsafe methods in interfaces

src/Microsoft.Cci.Extensions/Writers/CSharp/CSDeclarationWriter.Methods.cs

@@ -33,7 +33,14 @@ private void WriteMethodDefinition(IMethodDefinition method)
                 return;
             }
 
-            if (!method.ContainingTypeDefinition.IsInterface)
+            if (method.ContainingTypeDefinition.IsInterface)
+            {
+                if (method.IsMethodUnsafe())
+                {
+                    WriteKeyword("unsafe");
+                }
+            }
+            else
             {
                 if (!method.IsExplicitInterfaceMethod() && !method.IsStaticConstructor)
                 {
@@ -42,6 +49,7 @@ private void WriteMethodDefinition(IMethodDefinition method)
 
                 WriteMethodModifiers(method);
             }
+
             WriteInterfaceMethodModifiers(method);
             WriteMethodDefinitionSignature(method);
             WriteMethodBody(method);
@@ -245,8 +253,11 @@ private void WriteInterfaceMethodModifiers(IMethodDefinition method)
 
         private void WriteMethodModifiers(IMethodDefinition method)
         {
-            if (method.IsMethodUnsafe())
+            if (method.IsMethodUnsafe() ||
+                (method.IsConstructor && IsBaseConstructorCallUnsafe(method.ContainingTypeDefinition)))
+            {
                 WriteKeyword("unsafe");
+            }
 
             if (method.IsStatic)
                 WriteKeyword("static");
@@ -344,6 +355,11 @@ private void WritePrivateConstructor(ITypeDefinition type)
                 return;
 
             WriteVisibility(TypeMemberVisibility.Assembly);
+            if (IsBaseConstructorCallUnsafe(type))
+            {
+                WriteKeyword("unsafe");
+            }
+
             WriteIdentifier(((INamedEntity)type).Name);
             WriteSymbol("(");
             WriteSymbol(")");
@@ -353,24 +369,7 @@ private void WritePrivateConstructor(ITypeDefinition type)
 
         private void WriteBaseConstructorCall(ITypeDefinition type)
         {
-            if (!_forCompilation)
-                return;
-
-            ITypeDefinition baseType = type.BaseClasses.FirstOrDefault().GetDefinitionOrNull();
-
-            if (baseType == null)
-                return;
-
-            var ctors = baseType.Methods.Where(m => m.IsConstructor && _filter.Include(m) && !m.Attributes.Any(a => a.IsObsoleteWithUsageTreatedAsCompilationError()));
-
-            var defaultCtor = ctors.Where(c => c.ParameterCount == 0);
-
-            // Don't need a base call if we have a default constructor
-            if (defaultCtor.Any())
-                return;
-
-            var ctor = ctors.FirstOrDefault();
-
+            var ctor = GetBaseConstructorForCall(type);
             if (ctor == null)
                 return;
 
@@ -382,6 +381,52 @@ private void WriteBaseConstructorCall(ITypeDefinition type)
             WriteSymbol(")");
         }
 
+        private bool IsBaseConstructorCallUnsafe(ITypeDefinition type)
+        {
+            var constructor = GetBaseConstructorForCall(type);
+            if (constructor == null)
+            {
+                return false;
+            }
+
+            foreach (var parameter in constructor.Parameters)
+            {
+                if (parameter.Type.IsUnsafeType())
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        private IMethodDefinition GetBaseConstructorForCall(ITypeDefinition type)
+        {
+            if (!_forCompilation)
+            {
+                // No need to generate a call to a base constructor.
+                return null;
+            }
+
+            var baseType = type.BaseClasses.FirstOrDefault().GetDefinitionOrNull();
+            if (baseType == null)
+            {
+                // No base type to worry about.
+                return null;
+            }
+
+            var constructors = baseType.Methods.Where(
+                m => m.IsConstructor && _filter.Include(m) && !m.Attributes.Any(a => a.IsObsoleteWithUsageTreatedAsCompilationError()));
+
+            if (constructors.Any(c => c.ParameterCount == 0))
+            {
+                // Don't need a base call if base class has a default constructor.
+                return null;
+            }
+
+            return constructors.FirstOrDefault();
+        }
+
         /// <summary>
         /// When generated .notsupported.cs files, we need to generate calls to the base constructor.
         /// However, if the base constructor doesn't accept null, passing default(T) will cause a compile