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

Question

In using the EntityFramework  I get the error  A lambda expression with a statement body cannot be converted to an expression tree  when trying to compile the following code   Obj   myArray   objects Select o   gt        var someLocalVar   o someVar       return new Obj          Var1   someLocalVar      Var2   o var2       ToArray      I don t know what the error means and most of all how to fix it  Any help

User · Answer

You can use statement body in lamba expression for IEnumerable collections. try this one:

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

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

Notice:
Think carefully when using this method, because this way, you will have all query result in memory, that may have unwanted side effects on the rest of your code.

User · Answer

Use this overload of select   Obj   myArray   objects Select new Func lt Obj Obj gt   o   gt        var someLocalVar   o someVar       return new Obj                  Var1   someLocalVar         Var2   o var2             ToArray

User · Answer

It means that you can t use lambda expressions with a  statement body   i e  lambda expressions which use curly braces  in places where the lambda expression needs to be converted to an expression tree  which is for example the case when using linq2sql

User · Answer

9 years too late to the party  but a different approach to your problem  that nobody has mentioned    The statement-body works fine with Func lt  gt  but won t work with Expression lt Func lt  gt  gt   IQueryable Select wants an Expression lt  gt   because they can be translated for Entity Framework - Func lt  gt  can not  So you either use the AsEnumerable and start working with the data in memory  not recommended  if not really neccessary  or you keep working with the IQueryable lt  gt  which is recommended  There is something called linq query which makes some things easier  IQueryable lt Obj gt  result   from o in objects                          let someLocalVar   o someVar                          select new Obj                                                       Var1   someLocalVar                             Var2   o var2                              with let you can define a variable and use it in the select  or where      - and you keep working with the IQueryable until you really need to execute and get the objects  Afterwards you can Obj   myArray   result ToArray

User · Answer

The LINQ to SQL return object were implementing IQueryable interface  So for Select method predicate parameter you should only supply single lambda expression without body   This is because LINQ for SQL code is not execute inside program rather than on remote side like SQL server or others  This lazy loading execution type were achieve by implementing IQueryable where its expect delegate is being wrapped in Expression type class like below   Expression lt Func lt TParam TResult gt  gt    Expression tree do not support  lambda expression with body and its only support single line lambda expression like var id   cols Select  col   gt  col id     So if you try the following code won t works   Expression lt Func lt int int gt  gt  function   x   gt        return x   2      The following will works as per expected   Expression lt Func lt int int gt  gt  function   x   gt  x   2

User · Answer

Without knowing more about what you are doing  Linq2Objects  Linq2Entities  Linq2Sql    this should make it work   Arr   myArray   objects AsEnumerable   Select o   gt        var someLocalVar   o someVar       return new Obj              Var1   someLocalVar          Var2   o var2             ToArray

User · Answer

It means that a Lambda expression of type TDelegate which contains a   parameters     gt    some code    cannot be converted to an Expression lt TDelegate gt    It s the rule   Simplify your query   The one you provided can be rewritten as the following and will compile   Arr   myArray   objects Select o   gt  new Obj                                        Var1   o someVar                     Var2   o var2                     ToArray

User · Answer

Is Arr a base type of Obj  Does the Obj class exist  Your code would work only if Arr is a base type of Obj  You can try this instead   Obj   myArray   objects Select o   gt        var someLocalVar   o someVar       return new Obj                  Var1   someLocalVar         Var2   o var2            ToArray

User · Answer

For your specific case  the body is for creating a variable  and switching to IEnumerable will force all the operations to be processed on client-side  I propose the following solution   Obj   myArray   objects  Select o   gt  new       SomeLocalVar   o someVar     You can even use any LINQ statement here     Info   o     Select o   gt  new Obj         Var1   o SomeLocalVar      Var2   o Info var2      Var3   o SomeLocalVar SubValue1      Var4   o SomeLocalVar SubValue2     ToArray      Edit  Rename for C  Coding Convention

User · Answer

Is objects a Linq-To-SQL database context  In which case  you can only use simple expressions to the right of the    operator  The reason is  these expressions are not executed  but are converted to SQL to be executed against the database  Try this  Arr   myArray   objects Select o   gt  new Obj          Var1   o someVar      Var2   o var2     ToArray

[c#] "A lambda expression with a statement body cannot be converted to an expression tree"

Examples related to c#

Examples related to linq

Examples related to entity-framework

Examples related to linq-to-entities