Why Does My Lambda Expression with a Statement Body Cause an Error in Entity Framework?

Lambda Expression with Statement Body Error in EntityFramework

In EntityFramework, a common error encountered when working with lambda expressions is:

"A lambda expression with a statement body cannot be converted to an expression tree."

This error occurs when a lambda expression contains statements instead of an expression. In EntityFramework, lambda expressions are used to specify conditions or transformations within queries. However, expressions cannot contain statements that modify variables or perform actions.

Example:

The following code snippet triggers the error:

Obj[] myArray = objects.Select(o =>
{
    var someLocalVar = o.someVar;

    return new Obj() { 
    Var1 = someLocalVar,
    Var2 = o.var2 };
}).ToArray();

Explanation:

In this example, the lambda expression uses a statement body to assign a value to the local variable someLocalVar before returning an object. However, this statement body cannot be converted to an expression tree for execution by EntityFramework.

Solution:

To resolve this error, rewrite the lambda expression so that it only contains expressions. In this case, this means removing the statement body and directly constructing the Obj object:

Arr[] myArray = objects.Select(o => new Obj() { 
    Var1 = o.someVar,
    Var2 = o.var2 
}).ToArray();

Now the lambda expression consists solely of an expression and can be converted to an expression tree for processing by EntityFramework.

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