How do I create an expression tree to represent 'String.Contains("term")' in C#?

Question

I am just getting started with expression trees so I hope this makes sense. I am trying to create an expression tree to represent:

t => t.SomeProperty.Contains("stringValue"); 

So far I have got:

    private static Expression.Lambda<Func<string, bool>> GetContainsExpression<T>(string propertyName, string propertyValue)     {         var parameterExp = Expression.Parameter(typeof(T), "type");         var propertyExp = Expression.Property(parameter, propertyName);         var containsMethodExp = Expression.*SomeMemberReferenceFunction*("Contains", propertyExp) //this is where I got lost, obviously :)         ...         return Expression.Lambda<Func<string, bool>>(containsMethodExp, parameterExp); //then something like this     } 

I just don't know how to reference the String.Contains() method.

Help appreciated.

Answer 1

Something like:

class Foo {     public string Bar { get; set; } } static void Main() {     var lambda = GetExpression<Foo>("Bar", "abc");     Foo foo = new Foo { Bar = "aabca" };     bool test = lambda.Compile()(foo); } static Expression<Func<T, bool>> GetExpression<T>(string propertyName, string propertyValue) {     var parameterExp = Expression.Parameter(typeof(T), "type");     var propertyExp = Expression.Property(parameterExp, propertyName);     MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });     var someValue = Expression.Constant(propertyValue, typeof(string));     var containsMethodExp = Expression.Call(propertyExp, method, someValue);      return Expression.Lambda<Func<T, bool>>(containsMethodExp, parameterExp); } 

You might find this helpful.

Answer 2

To perform a search like:

ef.Entities.Where(entity => arr.Contains(entity.Name)).ToArray(); 

which the trace string will be:

SELECT .... From Entities ... Where Name In ("abc", "def", "qaz") 

I use the method I created below:

ef.Entities.Where(ContainsPredicate<Entity, string>(arr, "Name")).ToArray();  public Expression<Func<TEntity, bool>> ContainsPredicate<TEntity, T>(T[] arr, string fieldname) where TEntity : class {   ParameterExpression entity = Expression.Parameter(typeof(TEntity), "entity");   MemberExpression member = Expression.Property(entity, fieldname);    var containsMethods = typeof(Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public)   .Where(m => m.Name == "Contains");   MethodInfo method = null;   foreach (var m in containsMethods) {     if (m.GetParameters().Count() == 2) {       method = m;       break;     }   }   method = method.MakeGenericMethod(member.Type);   var exprContains = Expression.Call(method, new Expression[] { Expression.Constant(arr), member });   return Expression.Lambda<Func<TEntity, bool>>(exprContains, entity); }