Why Does Entity Framework Throw 'Lambda Expression with Statement Body Cannot Be Converted to Expression Tree'?

Compiler Error: "Lambda Expression with Statement Body Cannot Be Converted to Expression Tree"

When working with EntityFramework, you may encounter an error stating, "A lambda expression with a statement body cannot be converted to an expression tree." This error occurs when you attempt to use a lambda expression with a block statement (also known as a statement lambda) in a context where only expression lambdas are allowed.

The Issue

As the error message suggests, EntityFramework expects expression lambdas, which are concise expressions without any block statements. In the example provided, the lambda expression:

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

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

contains a block statement, making it a statement lambda. EntityFramework cannot convert this expression to a form suitable for executing database queries.

The Solution

To resolve the error, refactor your lambda expression to use an expression lambda. Expression lambdas are one-line expressions that directly return a value without using any additional statements. In this case, you can rewrite the lambda as follows:

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

This expression lambda eliminates the block statement and directly returns a new Obj object. This form is compatible with EntityFramework's expectation of expression trees and will resolve the compilation error.

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