Expressions call constructor with a parameter and set its value

c# constructor expression-trees lambda reflection


i am trying to call an parameterized constructor from an expression instead of using the default ctor. this is the code that gets the constructor parameter(s):

ConstructorInfo ci = type.GetConstructor(BindingFlags.Instance | BindingFlags.Public, null, CallingConventions.HasThis, new[] { typeof(bool) }, new ParameterModifier[] { });
ParameterInfo[] paramsInfo = ci.GetParameters();

//create a single param of type object[]
ParameterExpression param = Expression.Parameter(typeof(bool), "il");

Expression[] argsExp = new Expression[paramsInfo.Length];

//pick each arg from the params array 
//and create a typed expression of them
for (int i = 0; i < paramsInfo.Length; i++)
    Expression index = Expression.Constant(i);
    Type paramType = paramsInfo[i].ParameterType;

    Expression paramAccessorExp = param;
    //Expression.ArrayIndex(param, index);

    Expression paramCastExp =
        Expression.Convert(paramAccessorExp, paramType);

    argsExp[i] = param;

NewExpression ci2 = Expression.New(ci, argsExp);

But if i try to compile the lambda expression i am getting the following error:

variable 'il' of type 'System.Boolean' referenced from scope '', but it is not defined"

What am i missing? Any help and/ or hint is appreciated.

Accepted Answer

You define a parameter called li in the 4th line of your code. In order to use this in a lambda expression, you need to have a scope in which this parameter is defined. You have two choices:

  1. Create a BlockExpression that contains param as a local variable. Then use this expression as the body of your lambda expression.
  2. Use param as a parameter in your LambdaExpression.

If you use option 1, you'll also have to initialize the variable. Otherwise you'll get a different kind of error message.


There are two problems with the additional code you posted:

  1. You need to use the same parameter object throughout your expression tree. Having the same name and type does not make two Parameter objects equal. I would simply move everything up to and including creating the lambda to the ConvertThis method so you can reuse the param variable. You can then just compile the return value of ConvertThis to get your delegate.

  2. When creating the BlockExpression, you need to pass param in as a local variable. You do this by adding an argument, new ParameterExpression[] { param } to the method.

Popular Answer


ParameterExpression param = Expression.Parameter(typeof(bool); 

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