How to manually build Expression which will return always true?

c# expression-trees lambda linq

Question

I tried to create Expression, but failed.

I want to build something like Expression<Func<typeof(type), bool>> expression = _ => true;

My attempt:

private static Expression GetTrueExpression(Type type)
{
    LabelTarget returnTarget = Expression.Label(typeof(bool));
    ParameterExpression parameter = Expression.Parameter(type, "x");

    var resultExpression = 
      Expression.Return(returnTarget, Expression.Constant(true), typeof(bool));

    var delegateType = typeof(Func<,>).MakeGenericType(type, typeof(bool));

    return Expression.Lambda(delegateType, resultExpression, parameter); ;
}

Usage:

var predicate = Expression.Lambda(GetTrueExpression(typeof(bool))).Compile();

But I am getting the error: Cannot jump to undefined label ''

1
5
4/21/2016 3:05:27 PM

Accepted Answer

As simple as that:

private static Expression GetTrueExpression(Type type)
{
    return Expression.Lambda(Expression.Constant(true), Expression.Parameter(type, "_"));
}
11
4/21/2016 3:02:53 PM

Popular Answer

There is a nice tool which can help you construct expression trees http://tryroslyn.azurewebsites.net/ .

I've added a linq to @Ivan`s example of GetTrueExpression



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