Question

I need to generate a lambda expression like

item => item.Id > 5 && item.Name.StartsWith("Dish")

Ok, item.Id > 5 is simple

var item = Expression.Parameter(typeof(Item), "item");

var propId = Expression.Property(item,"Id");
var valueId = Expression.Constant(5);
var idMoreThanFive = Expression.GreaterThan(propId, valueId);

But the second part is more complex for me…

var propName = Expression.Property(item,"Name");
var valueName = Expression.Constant("Dish");

How to call StartsWith for propName?

Answer 1

You’ll have to get a MethodInfo representing the string.StartsWith(string) method and then use Expression.Call to construct the expression representing the instancemethod call:

var property = Expression.Property(item, "Name");
var method = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
var argument = Expression.Constant("Dish");

// item.Name.StartsWith("Dish")
var startsWithDishExpr = Expression.Call(property, method, argument);

You’ll then have to && the subexpressions together to create the body.

var lambdaBody = Expression.AndAlso(idMoreThanFive, startsWithDishExpr);

And then finally construct the lambda:

var lambda = Expression.Lambda<Func<Item, bool>>(lambdaBody, item);