Replace lambda expression in MongoDB query with Func

Question

I would like to replace the lambda expression in the following code

var idQuery = Query<ApiStatisticsAggregatedStats>.EQ(t => t.Api, id);

with a func. Hovering over the statement it says the declaration is

Func<ApiStatisticsAggregatedStats, string>

The following code gives an error though,

Func<ApiStatisticsAggregatedStats, string> idFunc = x => x.Api
var idQuery = Query<ApiStatisticsAggregatedStats>.EQ(idFunc, id);

Error:

The best overloaded method match for Query<ApiStatisticsAggregatedStats>.EQ<string>(Expression<Func<ApiStatisticsAggregatedStats, IEnumerable<string>>>, string) has some invalid arguments.

Where did that IEnumerable come from? What am I doing wrong?

@marc_s Thanks for cleaning up the post. However, that error section you did truncated the message since the "less-than" characters were removed. — user1323245
– user1323245, Commented Mar 25, 2014 at 16:34

MarcinJuraszek · Accepted Answer · 2014-03-25 16:37:44Z

5

Query providers, like mongoDB query provider, use expression trees instead of delegates. That's why you have to declare your variable as

Expression<Func<ApiStatisticsAggregatedStats, string>> idFunc = x => x.Api

You can still assign a lambda expression to Expression<Func<...>> variable, because compiler can transform your lambda into proper method calls from Expression class during compilation.

When a lambda expression is assigned to a variable of type Expression<TDelegate>, the compiler emits code to build an expression tree that represents the lambda expression.

However, it works for single-line lambdas only.

The C# and Visual Basic compilers can generate expression trees only from expression lambdas (or single-line lambdas). It cannot parse statement lambdas (or multi-line lambdas).

If you'd try doing following:

Expression<Func<ApiStatisticsAggregatedStats, string>> idFunc = x => { return x.Api };

(notice { and }) you'd get compile error saying compiler is not able to transform multi-line lambda into expression tree.

Bot quotes come from Expression Trees (C# and Visual Basic).

answered Mar 25, 2014 at 16:37

MarcinJuraszek

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1 Comment

user1323245 Over a year ago

Awesome. Worked wonders. I had tried to assign the Func to an Expression, which compiled but failed at runtime. Guess it's time to properly read through the section on expression trees in C# in Depth.

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