internal CatchBlock Reduce(Action <ParameterExpression> hoistVariable)
{
// NB: System.Linq.Expressions does not support locals that cross the filter and the body of a CatchBlock, so we have to
// hoist these variables to a surrounding scope. In order to avoid shadowing other uses of the same variable object,
// e.g. in the try block, in another catch block, or in the finally block, we perform variable substitution in the
// filter and the body expression in order to guarantee that the hoisted variable is unique.
//
// For example:
//
// Block({ a }, {
// Try { /* use a */ }
// Catch (e) When (e is A a) { /* use a */ }
// })
//
// If we hoist 'a' from the 'When' clause to a new block surrounding 'Try', it will shadow the 'a' from the outer
// block and change the meaning of the try block's usage of 'a'.
//
// Block({ a }, {
// Block({ a }, { // BUG
// Try { /* use a */ } // BUG
// Catch (e) When (e is A a) { /* use a */ }
// })
// })
//
// Instead, we substitute the variables associated with the Catch block using new ones which are thus guaranteed
// not to be used elsewhere and are safe for hoisting.
//
// Block({ a }, {
// Block({ t }, { // FIX
// Try { /* use a */ } // OKAY, refers to the original 'a'
// Catch (e) When (e is A t) { /* use t */ } // FIX, we introduce 't' in the catch clause subexpressions
// })
// })
var filter = Filter;
var body = Body;
if (Variables.Count > 0)
{
var variablesToRename = new Dictionary <ParameterExpression, Expression>();
foreach (var variable in Variables)
{
if (variable != Variable) // NB: Variable gets its own scope by CatchBlock so does not need hosting.
{
var newVariable = Expression.Parameter(variable.Type, variable.Name);
variablesToRename.Add(variable, newVariable);
hoistVariable(newVariable);
}
}
body = ParameterSubstitutor.Substitute(body, variablesToRename);
filter = ParameterSubstitutor.Substitute(filter, variablesToRename);
}
return(Expression.MakeCatchBlock(Test, Variable, body, filter));
}
public void ParameterSubstitutor_Basics()
{
var x = Expression.Parameter(typeof(int));
var y = Expression.Parameter(typeof(int));
var e1 = Expression.Add(x, Expression.Constant(1));
var r1 = (BinaryExpression)ParameterSubstitutor.Substitute(e1, x, y);
Assert.AreSame(y, r1.Left);
var e2 = Expression.Block(new[] { x }, x);
var r2 = (BlockExpression)ParameterSubstitutor.Substitute(e2, x, y);
Assert.AreSame(x, r2.Expressions[0]);
var e3 = Expression.Block(Expression.Add(x, Expression.Constant(3)));
var r3 = (BinaryExpression)((BlockExpression)ParameterSubstitutor.Substitute(e3, x, y)).Expressions[0];
Assert.AreSame(y, r3.Left);
}
protected Expression ReduceSingle(ParameterExpression variable, Expression resource, Expression body, HashSet <ParameterExpression> declaredVariables)
{
var madeTempVariable = false;
void MakeTempIfNull(ref ParameterExpression variable, Type type)
{
if (variable == null)
{
variable = Expression.Parameter(type, "__resource");
madeTempVariable = true;
}
else
{
declaredVariables.Add(variable);
}
}
Expression CallPatternDispose(Expression nonNullResource)
{
return(ParameterSubstitutor.Substitute(PatternDispose.Body, PatternDispose.Parameters[0], nonNullResource));
}
Expression cleanup;
var checkNull = false;
var resourceType = resource.Type;
if (resourceType.IsValueType)
{
MakeTempIfNull(ref variable, resourceType);
Expression variableValue;
if (resourceType.IsNullableType())
{
variableValue = Helpers.MakeNullableGetValueOrDefault(variable);
checkNull = true;
}
else
{
variableValue = variable;
}
if (PatternDispose != null)
{
cleanup = CallPatternDispose(variableValue);
}
else
{
var disposeMethod = variableValue.Type.FindDisposeMethod(IsAsync);
cleanup = Expression.Call(variableValue, disposeMethod);
}
}
else
{
var disposableInterface = IsAsync ? typeof(IAsyncDisposable) : typeof(IDisposable);
MakeTempIfNull(ref variable, disposableInterface);
// NB: This optimization would be more effective if the expression compiler would emit a `call` instruction,
// but the JIT may still optimize it if it realizes the `callvirt` to the resource is predicated by a
// prior null check.
var variableType = variable.Type;
if (PatternDispose != null)
{
cleanup = CallPatternDispose(variable);
}
else
{
var disposeMethod =
variableType.IsSealed
? variableType.FindDisposeMethod(IsAsync)
: (IsAsync ? DisposeAsyncMethod : DisposeMethod);
cleanup = Expression.Call(variable, disposeMethod);
}
checkNull = true;
}
if (IsAsync)
{
cleanup = Await(cleanup);
}
if (checkNull)
{
cleanup =
Expression.IfThen(
Expression.NotEqual(variable, Expression.Constant(null, variable.Type)),
cleanup
);
}
ParameterExpression temp = default;
Expression innerResource;
if (!madeTempVariable)
{
// NB: Resource could contain a reference to Variable that needs to be bound in the
// enclosing scope. This isn't possible to write in C# due to scoping rules for
// variables, but it's valid in the LINQ APIs in general.
//
// +-------------+
// v |
// Block({ x }, Using(x, R(x), Call(x, foo)))
// ^ |
// +---------------+
//
// If we're not careful about scoping, we could end up creating:
//
// +-------------+
// v |
// Block({ x }, Using(x, R(x), Call(x, foo)))
// ^ |
// +----+
//
// So we rewrite the whole thing by adding another temporary variable:
//
// +----------+
// v |
// Block({ x }, Block({ t }, Assign(t, R(x)), Using(x, t, Call(x, foo))))
// ^ |
// +-----------------------------+
//
// NB: We could do a scope tracking visit to Resource to check whether the variable
// is being referred to. For now, we'll just apply the additional assignment all
// the time, but we could optimize this later (or we could invest in a /o+ type
// of flag for all of the expression APIs, so we can optimize the user's code as
// well when it exhibits patterns like this; additional Blocks seems common when
// generating code from extension nodes of a higher abstraction kind).
temp = Expression.Parameter(variable.Type, "__temp");
innerResource = temp;
}
else
{
innerResource = resource;
}
var res =
Expression.Block(
new[] { variable },
Expression.Assign(variable, innerResource),
Expression.TryFinally(
body,
cleanup
)
);
if (temp != null)
{
// NB: See remarks above for an explation of the need for this additional scope.
res =
Expression.Block(
new[] { temp },
Expression.Assign(temp, resource),
res
);
}
return(res);
}