private BoundExpression MakeNullCoalescingOperator(
SyntaxNode syntax,
BoundExpression rewrittenLeft,
BoundExpression rewrittenRight,
Conversion leftConversion,
BoundNullCoalescingOperatorResultKind resultKind,
TypeSymbol?rewrittenResultType)
{
Debug.Assert(rewrittenLeft != null);
Debug.Assert(rewrittenRight != null);
Debug.Assert(leftConversion.IsValid);
Debug.Assert(rewrittenResultType is { });
private BoundExpression MakeNullCoalescingOperator(
SyntaxNode syntax,
BoundExpression rewrittenLeft,
BoundExpression rewrittenRight,
BoundValuePlaceholder?leftPlaceholder,
BoundExpression?leftConversion,
BoundNullCoalescingOperatorResultKind resultKind,
TypeSymbol?rewrittenResultType)
{
Debug.Assert(rewrittenLeft != null);
Debug.Assert(rewrittenRight != null);
Debug.Assert(BoundNode.GetConversion(leftConversion, leftPlaceholder).IsValid);
Debug.Assert(rewrittenResultType is { });
private BoundExpression MakeNullCoalescingOperator(
SyntaxNode syntax,
BoundExpression rewrittenLeft,
BoundExpression rewrittenRight,
Conversion leftConversion,
BoundNullCoalescingOperatorResultKind resultKind,
TypeSymbol rewrittenResultType)
{
Debug.Assert(rewrittenLeft != null);
Debug.Assert(rewrittenRight != null);
Debug.Assert(leftConversion.IsValid);
Debug.Assert((object)rewrittenResultType != null);
Debug.Assert(rewrittenRight.Type.Equals(rewrittenResultType, TypeCompareKind.IgnoreDynamicAndTupleNames | TypeCompareKind.IgnoreNullableModifiersForReferenceTypes));
var isUnconstrainedTypeParameter = (object)rewrittenLeft.Type != null && !rewrittenLeft.Type.IsReferenceType && !rewrittenLeft.Type.IsValueType;
// first we can make a small optimization:
// If left is a constant then we already know whether it is null or not. If it is null then we
// can simply generate "right". If it is not null then we can simply generate
// MakeConversion(left). This does not hold when the left is an unconstrained type parameter: at runtime,
// it can be either left or right depending on the runtime type of T
if (!isUnconstrainedTypeParameter)
{
if (rewrittenLeft.IsDefaultValue())
{
return(rewrittenRight);
}
if (rewrittenLeft.ConstantValue != null)
{
Debug.Assert(!rewrittenLeft.ConstantValue.IsNull);
return(GetConvertedLeftForNullCoalescingOperator(rewrittenLeft, leftConversion, rewrittenResultType));
}
}
// string concatenation is never null.
// interpolated string lowering may introduce redundant null coalescing, which we have to remove.
if (IsStringConcat(rewrittenLeft))
{
return(GetConvertedLeftForNullCoalescingOperator(rewrittenLeft, leftConversion, rewrittenResultType));
}
// if left conversion is intrinsic implicit (always succeeds) and results in a reference type
// we can apply conversion before doing the null check that allows for a more efficient IL emit.
if (rewrittenLeft.Type.IsReferenceType &&
leftConversion.IsImplicit &&
!leftConversion.IsUserDefined)
{
if (!leftConversion.IsIdentity)
{
rewrittenLeft = MakeConversionNode(rewrittenLeft.Syntax, rewrittenLeft, leftConversion, rewrittenResultType, @checked: false);
}
return(new BoundNullCoalescingOperator(syntax, rewrittenLeft, rewrittenRight, Conversion.Identity, resultKind, rewrittenResultType));
}
if (leftConversion.IsIdentity || leftConversion.Kind == ConversionKind.ExplicitNullable)
{
var conditionalAccess = rewrittenLeft as BoundLoweredConditionalAccess;
if (conditionalAccess != null &&
(conditionalAccess.WhenNullOpt == null || NullableNeverHasValue(conditionalAccess.WhenNullOpt)))
{
var notNullAccess = NullableAlwaysHasValue(conditionalAccess.WhenNotNull);
if (notNullAccess != null)
{
var whenNullOpt = rewrittenRight;
if (whenNullOpt.Type.IsNullableType())
{
notNullAccess = conditionalAccess.WhenNotNull;
}
if (whenNullOpt.IsDefaultValue())
{
whenNullOpt = null;
}
return(conditionalAccess.Update(
conditionalAccess.Receiver,
conditionalAccess.HasValueMethodOpt,
whenNotNull: notNullAccess,
whenNullOpt: whenNullOpt,
id: conditionalAccess.Id,
type: rewrittenResultType
));
}
}
}
// Optimize left ?? right to left.GetValueOrDefault() when left is T? and right is the default value of T
if (rewrittenLeft.Type.IsNullableType() &&
RemoveIdentityConversions(rewrittenRight).IsDefaultValue() &&
rewrittenRight.Type.Equals(rewrittenLeft.Type.GetNullableUnderlyingType(), TypeCompareKind.AllIgnoreOptions) &&
TryGetNullableMethod(rewrittenLeft.Syntax, rewrittenLeft.Type, SpecialMember.System_Nullable_T_GetValueOrDefault, out MethodSymbol getValueOrDefault))
{
return(BoundCall.Synthesized(rewrittenLeft.Syntax, rewrittenLeft, getValueOrDefault));
}
// We lower left ?? right to
//
// var temp = left;
// (temp != null) ? MakeConversion(temp) : right
//
BoundAssignmentOperator tempAssignment;
BoundLocal boundTemp = _factory.StoreToTemp(rewrittenLeft, out tempAssignment);
// temp != null
BoundExpression nullCheck = MakeNullCheck(syntax, boundTemp, BinaryOperatorKind.NotEqual);
// MakeConversion(temp, rewrittenResultType)
BoundExpression convertedLeft = GetConvertedLeftForNullCoalescingOperator(boundTemp, leftConversion, rewrittenResultType);
Debug.Assert(convertedLeft.HasErrors || convertedLeft.Type.Equals(rewrittenResultType, TypeCompareKind.IgnoreDynamicAndTupleNames | TypeCompareKind.IgnoreNullableModifiersForReferenceTypes));
// (temp != null) ? MakeConversion(temp, LeftConversion) : RightOperand
BoundExpression conditionalExpression = RewriteConditionalOperator(
syntax: syntax,
rewrittenCondition: nullCheck,
rewrittenConsequence: convertedLeft,
rewrittenAlternative: rewrittenRight,
constantValueOpt: null,
rewrittenType: rewrittenResultType,
isRef: false);
Debug.Assert(conditionalExpression.ConstantValue == null); // we shouldn't have hit this else case otherwise
Debug.Assert(conditionalExpression.Type.Equals(rewrittenResultType, TypeCompareKind.IgnoreDynamicAndTupleNames | TypeCompareKind.IgnoreNullableModifiersForReferenceTypes));
return(new BoundSequence(
syntax: syntax,
locals: ImmutableArray.Create(boundTemp.LocalSymbol),
sideEffects: ImmutableArray.Create <BoundExpression>(tempAssignment),
value: conditionalExpression,
type: rewrittenResultType));
}
