Fixed issue with lambdas and System.Action #995
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I just realized there is a bigger problem with lambdas and |
exyi
requested changes
Apr 16, 2021
| private Expression CreateLambdaExpression(Expression body, ParameterExpression[] parameters) | ||
| { | ||
| // Check if we have type information available | ||
| if (expressionDepth == 1 && ExpectedType != null && ReflectionUtils.IsDelegate(ExpectedType)) |
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The ExpectedType is the top level expected type. However, the lambda may be used somewhere inside of the expression. ExpectedType might be string, while the expression is MyMethod(x => Prop = x) and it will lead to the same problem with Action and Func
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This PR fixes an issue when compiling lambda expressions that need to be assigned to
System.Action<...>. It is also necessary for the type inference feature #936.Current state
If user defines a lambda in a binding whose expected type is
Action<...>, current implementation fails for all non-voidexpressions. DotVVM throws compilation exception because it is unable to find an implicit conversion to convert fromFunc<...>toAction<...>. This is due to the fact that every expression that has non-voidreturn type gets compiled into aFunc<...>.This is different from C#, where it is possible to have the following:
The expression above would be compiled into
Func<string, string>in DotVVM because the assignment gets evaluated to a string.Proposed solution
It is necessary to propagate information about expected types to the
BindingExpressionBuilder. This type information can be then used insideExpressionBuildingVisitorto validate that the body is a valid statement and if so, make sure to generateAction<...>if required.Breaking change
Interface
IBindingExpressionBuilderwas altered by adding an optional parameter to itsParsemethod. Regardless, I do not think that this change could actually break someone.