CODEWITHSUNDEEP
c#.netreflectionexpression-trees
AwkwardCoder
AwkwardCoder

Reputation: 25631

Calling parameterised constructor using compiled expression

I'm trying to create a compiled expression delegate to call a constructor taking a single parameter, I'm receiving the following exception:

Additional information: variable 'value' of type 'MyType' referenced from scope '', but it is not defined

The code is as follows:

var constructorInfo = instanceType.GetConstructors().Skip(1).First();

ParameterExpression param = Expression.Parameter(genericArgument, "value");
Delegate constructorDelegate = Expression.Lambda(Expression.New(constructorInfo, new Expression[] { param })).Compile();

I believe I'm receving the exception because the parameter 'value' is not scoped inside an Expression.Block.

How do I scope the parameter & constructor expressions inside an Expression.Block?

Upvotes: 3

Views: 1849

Answers (1)

Markus
Markus

Reputation: 22456

In order to declare the parameter value, you need to also specify it when creating the Lambda expression (see this overload of the Expression.Lambda method). Up to now, you only create a parameterized lambda expression, but do not declare the parameters that are used in the expression. Changing your code should solve the issue:

var lambdaExpr = Expression.Lambda(Expression.New(constructorInfo, 
                                                  new Expression[] { param }), 
                                   param);
Delegate constructorDelegate = lambdaExpr.Compile();

Upvotes: 7

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