// To convert a null-check to pattern-matching, we should make sure of a few things:
//
// (1) The pattern variable may not be used before the point of declaration.
//
// {
// var use = t;
// if (x is T t) {}
// }
//
// (2) The pattern variable may not be used outside of the new scope which
// is determined by the parent statement.
//
// {
// if (x is T t) {}
// }
//
// var use = t;
//
// (3) The pattern variable may not be used before assignment in opposite
// branches, if any.
//
// {
// if (x is T t) {}
// var use = t;
// }
//
// We walk up the tree from the point of null-check and see if any of the above is violated.
private bool CanSafelyConvertToPatternMatching()
{
// Keep track of whether the pattern variable is definitely assigned when false/true.
// We start by the null-check itself, if it's compared with '==', the pattern variable
// will be definitely assigned when false, because we wrap the is-operator in a !-operator.
var defAssignedWhenTrue = _comparison.IsKind(SyntaxKind.NotEqualsExpression, SyntaxKind.IsExpression);
foreach (var current in _comparison.Ancestors())
{
// Checking for any conditional statement or expression that could possibly
// affect or determine the state of definite-assignment of the pattern variable.
switch (current.Kind())
{
case SyntaxKind.LogicalAndExpression when !defAssignedWhenTrue:
case SyntaxKind.LogicalOrExpression when defAssignedWhenTrue:
// Since the pattern variable is only definitely assigned if the pattern
// succeeded, in the following cases it would not be safe to use pattern-matching.
// For example:
//
// if ((x = o as string) == null && SomeExpression)
// if ((x = o as string) != null || SomeExpression)
//
// Here, x would never be definitely assigned if pattern-matching were used.
return(false);
case SyntaxKind.LogicalAndExpression:
case SyntaxKind.LogicalOrExpression:
// Parentheses and cast expressions do not contribute to the flow analysis.
case SyntaxKind.ParenthesizedExpression:
case SyntaxKind.CastExpression:
// Skip over declaration parts to get to the parenting statement
// which might be a for-statement or a local declaration statement.
case SyntaxKind.EqualsValueClause:
case SyntaxKind.VariableDeclarator:
case SyntaxKind.VariableDeclaration:
continue;
case SyntaxKind.LogicalNotExpression:
// The !-operator negates the definitive assignment state.
defAssignedWhenTrue = !defAssignedWhenTrue;
continue;
case SyntaxKind.ConditionalExpression:
var conditionalExpression = (ConditionalExpressionSyntax)current;
if (LocalFlowsIn(defAssignedWhenTrue
? conditionalExpression.WhenFalse
: conditionalExpression.WhenTrue))
{
// In a conditional expression, the pattern variable
// would not be definitely assigned in the opposite branch.
return(false);
}
return(CheckExpression(conditionalExpression));
case SyntaxKind.ForStatement:
var forStatement = (ForStatementSyntax)current;
if (forStatement.Condition is null || !forStatement.Condition.Span.Contains(_comparison.Span))
{
// In a for-statement, only the condition expression
// can make this definitely assigned in the loop body.
return(false);
}
return(CheckLoop(forStatement, forStatement.Statement, defAssignedWhenTrue));
case SyntaxKind.WhileStatement:
var whileStatement = (WhileStatementSyntax)current;
return(CheckLoop(whileStatement, whileStatement.Statement, defAssignedWhenTrue));
case SyntaxKind.IfStatement:
var ifStatement = (IfStatementSyntax)current;
var oppositeStatement = defAssignedWhenTrue
? ifStatement.Else?.Statement
: ifStatement.Statement;
if (oppositeStatement != null)
{
var dataFlow = _semanticModel.AnalyzeRequiredDataFlow(oppositeStatement);
if (dataFlow.DataFlowsIn.Contains(_localSymbol))
{
// Access before assignment is not safe in the opposite branch
// as the variable is not definitely assigned at this point.
// For example:
//
// if (o is string x) { }
// else { Use(x); }
//
return(false);
}
if (dataFlow.AlwaysAssigned.Contains(_localSymbol))
{
// If the variable is always assigned here, we don't need to check
// subsequent statements as it's definitely assigned afterwards.
// For example:
//
// if (o is string x) { }
// else { x = null; }
//
return(true);
}
}
if (!defAssignedWhenTrue &&
!_semanticModel.AnalyzeRequiredControlFlow(ifStatement.Statement).EndPointIsReachable)
{
// Access before assignment here is only valid if we have a negative
// pattern-matching in an if-statement with an unreachable endpoint.
// For example:
//
// if (!(o is string x)) {
// return;
// }
//
// // The 'return' statement above ensures x is definitely assigned here
// Console.WriteLine(x);
//
return(true);
}
return(CheckStatement(ifStatement));
}
switch (current)
{
case ExpressionSyntax expression:
// If we reached here, it means we have a sub-expression that
// does not guarantee definite assignment. We should make sure that
// the pattern variable is not used outside of the expression boundaries.
return(CheckExpression(expression));
case StatementSyntax statement:
// If we reached here, it means that the null-check is appeared in
// a statement. In that case, the variable would be actually in the
// scope in subsequent statements, but not definitely assigned.
// Therefore, we should ensure that there is no use before assignment.
return(CheckStatement(statement));
}
// Bail out for error cases and unhandled cases.
break;
}
return(false);
}