Lambda Expression to be used in Select() query
- by jameschinnock
Hi, I am trying to build a lambda expression, containing two assignments (as shown further down), that I can then pass to a Queryable.Select() method.
I am trying to pass a string variable into a method and then use that variable to build up the lambda expression so that I can use it in a LINQ Select query.
My reasoning behind it is that I have a SQL Server datasource with many column names, I am creating a charting application that will allow the user to select, say by typing in the column name, the actual column of data they want to view in the y-axis of my chart, with the x-axis always being the DateTime. Therefore, they can essentially choose what data they chart against the DateTime value (it’s a data warehouse type app).
I have, for example, a class to store the retrieved data in, and hence use as the chart source of:
public class AnalysisChartSource
{
public DateTime Invoicedate { get; set; }
public Decimal yValue { get; set; }
}
I have (purely experimentaly) built an expression tree for the Where clause using the String value and that works fine:
public void GetData(String yAxis)
{
using (DataClasses1DataContext db = new DataClasses1DataContext())
{
var data = this.FunctionOne().AsQueryable<AnalysisChartSource>();
//just to get some temp data in....
ParameterExpression pe = Expression.Parameter(typeof(AnalysisChartSource), "p");
Expression left = Expression.MakeMemberAccess(pe,
typeof(AnalysisChartSource).GetProperty(yAxis));
Expression right = Expression.Constant((Decimal)16);
Expression e2 = Expression.LessThan(left, right);
Expression expNew = Expression.New(typeof(AnalysisChartSource));
LambdaExpression le = Expression.Lambda(left, pe);
MethodCallExpression whereCall = Expression.Call(
typeof(Queryable), "Where", new Type[] { data.ElementType },
data.Expression,
Expression.Lambda<Func<AnalysisChartSource, bool>>(e2, new ParameterExpression[] { pe }));
}
}
However……I have tried a similar approach for the Select statement, but just can’t get it to work as I need the Select() to populate both X and Y values of the AnalysisChartSource class, like this:
.Select(c => new AnalysisChartSource
{ Invoicedate = c.Invoicedate, yValue = c.yValue}).AsEnumerable();
How on earth can I build such an expression tree….or….possibly more to the point…..is there an easier way that I have missed entirely?
