Why would you use Expression Func T rather than Func T

Question

I understand lambdas and the Func and Action delegates  But expressions  stump me    In what circumstances would you use an Expression lt Func lt T gt  gt  rather than a plain old Func lt T gt

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Overly simplified here  but Func is a machine  whereas Expression is a blueprint   D

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I m adding an answer-for-noobs because these answers seemed over my head  until I realized how simple it is  Sometimes it s your expectation that it s complicated that makes you unable to  wrap your head around it    I didn t need to understand the difference until I walked into a really annoying  bug  trying to use LINQ-to-SQL generically   public IEnumerable lt T gt  Get Func lt T  bool gt  conditionLambda     using var db   new DbContext         return db Set lt T gt  Where conditionLambda           This worked great until I started getting OutofMemoryExceptions on larger datasets  Setting breakpoints inside the lambda made me realize that it was iterating through each row in my table one-by-one looking for matches to my lambda condition  This stumped me for a while  because why the heck is it treating my data table as a giant IEnumerable instead of doing LINQ-to-SQL like it s supposed to  It was also doing the exact same thing in my LINQ-to-MongoDb counterpart   The fix was simply to turn Func lt T  bool gt  into Expression lt Func lt T  bool gt  gt   so I googled why it needs an Expression instead of Func  ending up here   An expression simply turns a delegate into a data about itself  So a   gt  a   1 becomes something like  On the left side there s an int a  On the right side you add 1 to it   That s it  You can go home now  It s obviously more structured than that  but that s essentially all an expression tree really is--nothing to wrap your head around   Understanding that  it becomes clear why LINQ-to-SQL needs an Expression  and a Func isn t adequate  Func doesn t carry with it a way to get into itself  to see the nitty-gritty of how to translate it into a SQL MongoDb other query  You can t see whether it s doing addition or multiplication or subtraction  All you can do is run it  Expression  on the other hand  allows you to look inside the delegate and see everything it wants to do  This empowers you to translate the delegate into whatever you want  like a SQL query  Func didn t work because my DbContext was blind to the contents of the lambda expression  Because of this  it couldn t turn the lambda expression into SQL  however  it did the next best thing and iterated that conditional through each row in my table   Edit  expounding on my last sentence at John Peter s request   IQueryable extends IEnumerable  so IEnumerable s methods like Where   obtain overloads that accept Expression  When you pass an Expression to that  you keep an IQueryable as a result  but when you pass a Func  you re falling back on the base IEnumerable and you ll get an IEnumerable as a result  In other words  without noticing you ve turned your dataset into a list to be iterated as opposed to something to query  It s hard to notice a difference until you really look under the hood at the signatures

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There is a more philosophical explanation about it from Krzysztof Cwalina s book Framework Design Guidelines  Conventions  Idioms  and Patterns for Reusable  NET Libraries          Edit for non-image version      Most times you re going to want Func or Action if all that needs to happen is to run some code  You need Expression when the code needs to be analyzed  serialized  or optimized before it is run  Expression is for thinking about code  Func Action is for running it

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When you want to treat lambda expressions as expression trees and look inside them instead of executing them  For example  LINQ to SQL gets the expression and converts it to the equivalent SQL statement and submits it to server  rather than executing the lambda    Conceptually  Expression lt Func lt T gt  gt  is completely different from Func lt T gt   Func lt T gt  denotes a delegate which is pretty much a pointer to a method and Expression lt Func lt T gt  gt  denotes a tree data structure for a lambda expression  This tree structure describes what a lambda expression does rather than doing the actual thing  It basically holds data about the composition of expressions  variables  method calls       for example it holds information such as this lambda is some constant   some parameter   You can use this description to convert it to an actual method  with Expression Compile  or do other stuff  like the LINQ to SQL example  with it  The act of treating lambdas as anonymous methods and expression trees is purely a compile time thing   Func lt int gt  myFunc        gt  10     similar to  int myAnonMethod     return 10      will effectively compile to an IL method that gets nothing and returns 10   Expression lt Func lt int gt  gt  myExpression        gt  10    will be converted to a data structure that describes an expression that gets no parameters and returns the value 10    larger image  While they both look the same at compile time  what the compiler generates is totally different

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LINQ is the canonical example  for example  talking to a database   but in truth  any time you care more about expressing what to do  rather  than actually doing it  For example  I use this approach in the RPC stack of protobuf-net  to avoid code-generation etc  - so you call a method with   string result   client Invoke svc   gt  svc SomeMethod arg1  arg2           This deconstructs the expression tree to resolve SomeMethod  and the value of each argument   performs the RPC call  updates any ref out args  and returns the result from the remote call  This is only possible via the expression tree  I cover this more here   Another example is when you are building the expression trees manually for the purpose of compiling to a lambda  as done by the generic operators code

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An extremely important consideration in the choice of Expression vs Func is that IQueryable providers like LINQ to Entities can  digest  what you pass in an Expression  but will ignore what you pass in a Func  I have two blog posts on the subject   More on Expression vs Func with Entity Framework and Falling in Love with LINQ - Part 7  Expressions and Funcs  the last section

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The primary reason is when you don t want to run the code directly  but rather  want to inspect it  This can be for any number of reasons    Mapping the code to a different environment  ie  C  code to SQL in Entity Framework  Replacing parts of the code in runtime  dynamic programming or even plain DRY techniques  Code validation  very useful when emulating scripting or when doing analysis  Serialization - expressions can be serialized rather easily and safely  delegates can t Strongly-typed safety on things that aren t inherently strongly-typed  and exploiting compiler checks even though you re doing dynamic calls in runtime  ASP NET MVC 5 with Razor is a nice example

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I d like to add some notes about the differences between Func lt T gt  and Expression lt Func lt T gt  gt     Func lt T gt  is just a normal old-school MulticastDelegate  Expression lt Func lt T gt  gt  is a representation of lambda expression in form of expression tree  expression tree can be constructed through lambda expression syntax or through the API syntax  expression tree can be compiled to a delegate Func lt T gt   the inverse conversion is theoretically possible  but it s a kind of decompiling  there is no builtin functionality for that as it s not a straightforward process  expression tree can be observed translated modified through the ExpressionVisitor  the extension methods for IEnumerable operate with Func lt T gt   the extension methods for IQueryable operate with Expression lt Func lt T gt  gt     There s an article which describes the details with code samples   LINQ  Func lt T gt  vs  Expression lt Func lt T gt  gt    Hope it will be helpful

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You would use an expression when you want to treat your function as data and not as code   You can do this if you want to manipulate the code  as data    Most of the time if you don t see a need for expressions then you probably don t need to use one

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I don t see any answers yet that mention performance   Passing Func lt  gt s into Where   or Count   is bad   Real bad   If you use a Func lt  gt  then it calls the IEnumerable LINQ stuff instead of IQueryable  which means that whole tables get pulled in and then filtered   Expression lt Func lt  gt  gt  is significantly faster  especially if you are querying a database that lives another server

[c#] Why would you use Expression<Func<T>> rather than Func<T>?

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