Build dynamic filter expression

Question

Could you help please,

I want to build lamda for filtering list of objects. I want to have builder that will accept list of fields and values and return lambda to use later to filter the list.

I've done the following:

public class SurveyListFilterBuilder
{
     public static Func<SurveysQueryResultItem, bool> Build(string[] searchField, string[] searchFieldValue, LogicalOperation operation)
    {
        Expression resultExpression = Expression.Constant(true);

        for (var i = 0; i < searchField.Length; i++)
        {
            var field = searchField[i];
              var fieldFilterExpression = GetFieldFilterExpression(field, searchFieldValue[i]);

            if (operation == LogicalOperation.And)
                resultExpression = Expression.And(resultExpression, fieldFilterExpression);
            else if (operation == LogicalOperation.Or)
                resultExpression = Expression.And(resultExpression, fieldFilterExpression);
        }

        return Expression.Lambda<Func<SurveysQueryResultItem, bool>>(resultExpression).Compile();
    }

    private static Expression<Func<MyClass, bool>> GetFieldFilterExpression(string field, string fieldValue)
    {
        switch (field)
        {
            case "name":
                return x => x.Name.Contains(fieldValue);
            case "description":
                return x => x.Name.Contains(fieldValue);
                default:
                    throw new NotSupportedException();
        }
    }
}

But it doesn't work because it seems I apply ADD operator bool expression and expression of Func:

The binary operator And is not defined for the types 'System.Boolean' and 'System.Func`2[ConsoleApp1.MyClass,System.Boolean]'.'

The question is how can I fix it and achieve correct result?

Siderite Zackwehdex Siderite Zackwehdex · Accepted Answer · 2018-11-18T22:16:28

First of all, in the "description" case you still use .Name. Also, you perform Expression.And both for LogicalOperation.And and LogicalOperation.Or.

Second, you start from an Expression.Constant(true), which is actually equivalent to a bool, while your return expressions are of Func<bool>. Your resultExpression must also be an Expression<Func<bool>> to use Add and Or.

Build dynamic filter expression

1 Answers