C# Select query not modifying external variable

问题

I have a class with n elements, and a property that returns the root sum square of the elements:

public double Length 
{ 
    get 
    { 
        double sum = 0.0; 
        Elements.Select(t => sum += t * t);  
        return Math.Sqrt(sum); 
    } 
}

However, that doesn't work - no matter the value of the elements, sum remains 0.0.
Why doesn't this work?

Note: I have already implemented it another way, but am looking to understand why the above code does not work

回答1:

LINQ uses deferred execution – the Select Method doesn't execute the lambda for all elements immediately, but returns an IEnumerable<T> which, when executed, performes the lambda on each element as it's enumerated.

Also note that LINQ is for querying, not for executing a block of code for each element. You should write your code such that there is no statement in the lambda, only a expression that is free of side-effects. You can use the Sum Method when you're trying to calculate a sum:

public double Length 
{ 
    get 
    { 
        double sum = elements.Select(t => t * t).Sum();
        return Math.Sqrt(sum); 
    } 
}

public double Length 
{ 
    get 
    { 
        double sum = elements.Sum(t => t * t);
        return Math.Sqrt(sum); 
    } 
}

回答2:

Deferred execution.

Try this:

public double Length
{
    get { return Math.Sqrt(Elements.Sum(t => t * t)); }
}

Here the linq query is executed immediately.

回答3:

This don't work because Deferred Execution. You can read this

LINQ and Deferred Execution

Understanding LINQ's Deferred Execution

来源：https://stackoverflow.com/questions/11287070/c-sharp-select-query-not-modifying-external-variable

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linq