Programmatically building a query with LINQ statement with entity framework

Question

Let's say I have a query like:

r = r.Where(x => iEnumerableMachineNames.Contains(x.Machine.Name) 
                 || x.Server==true);

Is there a way to build the predicate (I think that's what it's called) outside of the statement, for instance

t = (x => iEnumerableMachineNames.Contains(x.Machine.Name));
s = (x => x.Server==true)
q = t.Or(s);
r = r.Where(x => q);

Basically I want to programmatically build my query based on input parameters, for EF 5.

Moho · Accepted Answer · 2014-02-06 23:23:23Z

1

You can build expressions dynamically but not as simply as your pseudo code - it requires reflection and expression trees (read this).

A simple way to accomplish what it seems like you'd like to do is to short circuit different parts of the predicate using boolean flags:

bool testMachineName;
bool testIsServer;

r = r.Where( x =>
    ( !testMachineName || iEnumerableMachineNames.Contains( x.Machine.Name ) ) ||
    ( !testIsServer || x.Server ) );

answered Feb 6, 2014 at 23:23

Moho

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3 Comments

Matt Over a year ago

Would that work with EF or would it complain about not being able to construct that query? I guess I'll try it... :-)

Matt Over a year ago

Actually that doesn't work and returns everything right? If testMachine is false it's (true || whatever) == true and returns everything... Or am I going crazy?

Moho Over a year ago

that's why it's !testXyz - if you want a specific test, the expression will evaluate to false and force the remainder of the boolean expression to evaluate. If you don't want to test for x.Server == true, then you set testIsServer = false and that evaluates to true, thereby not filtering out items that are !x.Server

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