How to Append to an expression

Question

Based on my question from yesterday:

if I had to append to my existing \'where\' expression, how would i append?

Expression

        

Answer 1

I believe you can just do the following:

Expression> clientWhere = c => true;

if (filterByClientFName)
{
    var prefix = clientWhere.Compile();
    clientWhere = c => prefix(c) && c.ClientFName == searchForClientFName;
}
if (filterByClientLName)
{
    var prefix = clientWhere.Compile();
    clientWhere = c => prefix(c) && c.ClientLName == searchForClientLName;
}

If you need to keep everything in Expression-land (to use with IQueryable), you could also do the following:

Expression> clientWhere = c => true;

if (filterByClientFName)
{
    Expression> newPred = 
        c => c.ClientFName == searchForClientFName;
    clientWhere = Expression.Lambda>(
        Expression.AndAlso(clientWhere, newPred), clientWhere.Parameters);
}
if (filterByClientLName)
{
    Expression> newPred = 
        c => c.ClientLName == searchForClientLName;
    clientWhere = Expression.Lambda>(
        Expression.AndAlso(clientWhere, newPred), clientWhere.Parameters);
}

This can be made less verbose by defining this extension method:

public static Expression AndAlso(this Expression left, Expression right)
{
    return Expression.Lambda(Expression.AndAlso(left, right), left.Parameters);
}

You can then use syntax like this:

Expression> clientWhere = c => true;
if (filterByClientFName)
{
    clientWhere = clientWhere.AndAlso(c => c.ClientFName == searchForClientFName);
}
if (filterByClientLName)
{
    clientWhere = clientWhere.AndAlso(c => c.ClientLName == searchForClientLName);
}