I'm in the process of refactoring some code, attempting to make it more self-documenting. The current code has a query over an OData service which looks like this:
return context.MessageLog.Where
(
x =>
(
x.Status == MessageStatus.Success
|| x.Status == MessageStatus.Failure
)
&& x.Direction == MessageDirection.Inbound
&& x.ResponseDate == new DateTimeOffset(new DateTime(1900, 01, 01))
);
I'm hoping to change this to make use of Linq Expressions.
I could move all of the logic into a single expression and have code run context.MessageLog.Where(MessageIsPendingResponse);
.
However, I'd like to create expressions for the different conditions: MessageIsProcessed
(i.e. now at success or failure state), MessageIsInbound
and ResponseNotYetSent
(response date is null).
I could combine these with multiple where statements like so:
return context.MessageLog
.Where(MessageLogExpression.MessageIsProcessed)
.Where(MessageLogExpression.MessageIsInbound)
.Where(MessageLogExpression.ResponseNotYetSent);
(MessageLogExpression
being a class I use to contain these predefined expressions).
Is this the best way to combine the satements, or does it risk filtering on the wrong field first (e.g. does Linq combine all the conditions into a single query and allow the query engine (in SQL terms) to determine the best execution plan; or are we forcing it to filter on the Status field first?
The above is great for scenarios where we have an AND
joining our expressions; but how would we do an OR
?
I assume there's some way to combine these, but couldn't find anything obvious. I suspect something like this exists?
return context.MessageLog.Where(new OrExpression(MessageIsSuccess,MessageIsFailure));
Is there a good way to combine expressions within another expression defintion; e.g. something like the below code (only a version that compiles)?
public static Expression<Func<MessageLogRecord, bool>> MessageIsPendingResponse
{
get
{
Expression<Func<MessageLogRecord, bool>> expr = x =>
MessageIsProcessed(x)
&& MessageIsInbound(x)
&& ResponseNotYetSent(x);
return expr;
}
}
Addendum: Code for those expressions described above:
public class MessageLogExpression
{
public static Expression<Func<MessageLogRecord, bool>> MessageIsProcessed
{
get
{
Expression<Func<MessageLogRecord, bool>> expr = x =>
(
x.Status == MessageStatus.Success
|| x.Status == MessageStatus.Failure
);
return expr;
}
}
public static Expression<Func<MessageLogRecord, bool>> MessageIsInbound
{
get
{
Expression<Func<MessageLogRecord, bool>> expr = x =>
x.Direction == MessageDirection.Inbound;
return expr;
}
}
static readonly DateTimeOffset NullDate = new DateTimeOffset(new DateTime(1900, 01, 01));
public static Expression<Func<MessageLogRecord, bool>> ResponseNotYetSent
{
get
{
Expression<Func<MessageLogRecord, bool>> expr = x =>
x.ResponseDate == NullDate; //todo: test if this works or if I have to define the value within the expression
return expr;
}
}
}
About #1 - in terms of EF with Linq to Entities I'd expect that EF would create the same query regardless if I split it into multiple where conditions or have everything in one. Even if it won't - an SQL database might still produce the same query execution plan, since it has it's own optimizer.
About the other questions, we are using a helper class which is based on this blog post: http://www.albahari.com/nutshell/predicatebuilder.aspx
public static class PredicateBuilder
{
public static Expression<Func<T, bool>> True<T>()
{
return (Expression<Func<T, bool>>) (input => true);
}
public static Expression<Func<T, bool>> False<T>()
{
return (Expression<Func<T, bool>>) (input => false);
}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
{
InvocationExpression invocationExpression = Expression.Invoke((Expression) expression2, expression1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>((Expression) Expression.OrElse(expression1.Body, (Expression) invocationExpression), (IEnumerable<ParameterExpression>) expression1.Parameters);
}
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
{
InvocationExpression invocationExpression = Expression.Invoke((Expression) expression2, expression1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>((Expression) Expression.AndAlso(expression1.Body, (Expression) invocationExpression), (IEnumerable<ParameterExpression>) expression1.Parameters);
}
}
This works really nice as it helps you to combine expressions easily. You can start with PredicateBuilder.True<YourEntityHere>().And(... expression1 ...).And(...)...
if you want to merge OR expressions you do it similarly starting with false: PredicateBuilder.False<YourEntityHere>().Or(...)...
This means that for Q3 you can do:
public static Expression<Func<MessageLogRecord, bool>> MessageIsPendingResponse
{
get
{
Expression<Func<CCI_Int_ExportLog, bool>> expr = PredicateBuilder.True<MessageLogRecord>()
.And(MessageIsProcessed)
.And(MessageIsInbound)
.And(ResponseNotYetSent)
;
return expr;
}
}
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