LINQ : Dynamic select

Question

Consider we have this class :

    public  class Data
{
    public string Field1 { get; set; }
    public string Field2 { get; set; }
    public string Field         


        

Answer 1

I writing the method in following line for you can work with nested fields taking advantage of Nicholas Butler and Ali.

You can use this method for dynamically creating to lambda for pass to select and also works for nested fields. You can also work with IQueryable cases.

    /// 
    /// Format1: "Field1"
    /// Format2: "Nested1.Field1"
    /// Format3: "Field1:Field1Alias"
    /// 
    public static Expression> DynamicSelectGenerator(params string[] Fields)
    {
        string[] EntityFields = Fields;
        if (Fields == null || Fields.Length == 0)
            // get Properties of the T
            EntityFields = typeof(T).GetProperties().Select(propertyInfo => propertyInfo.Name).ToArray();

        // input parameter "x"
        var xParameter = Expression.Parameter(typeof(T), "x");

        // new statement "new Data()"
        var xNew = Expression.New(typeof(TSelect));

        // create initializers
        var bindings = EntityFields
            .Select(x =>
            {
                string[] xFieldAlias = x.Split(":");
                string field = xFieldAlias[0];

                string[] fieldSplit = field.Split(".");
                if (fieldSplit.Length > 1)
                {
                    // original value "x.Nested.Field1"
                    Expression exp = xParameter;
                    foreach (string item in fieldSplit)
                        exp = Expression.PropertyOrField(exp, item);

                    // property "Field1"
                    PropertyInfo member2 = null;
                    if (xFieldAlias.Length > 1)
                        member2 = typeof(TSelect).GetProperty(xFieldAlias[1]);
                    else
                        member2 = typeof(T).GetProperty(fieldSplit[fieldSplit.Length - 1]);

                    // set value "Field1 = x.Nested.Field1"
                    var res = Expression.Bind(member2, exp);
                    return res;
                }
                // property "Field1"
                var mi = typeof(T).GetProperty(field);
                PropertyInfo member;
                if (xFieldAlias.Length > 1)
                    member = typeof(TSelect).GetProperty(xFieldAlias[1]);
                else member = typeof(TSelect).GetProperty(field);

                // original value "x.Field1"
                var xOriginal = Expression.Property(xParameter, mi);

                // set value "Field1 = x.Field1"
                return Expression.Bind(member, xOriginal);
            }
        );

        // initialization "new Data { Field1 = x.Field1, Field2 = x.Field2 }"
        var xInit = Expression.MemberInit(xNew, bindings);

        // expression "x => new Data { Field1 = x.Field1, Field2 = x.Field2 }"
        var lambda = Expression.Lambda>(xInit, xParameter);

        return lambda;
    }

Usage:

var s = DynamicSelectGenerator(
            "Name:SalesTeamName",
            "Employee.FullName:SalesTeamExpert"
            );

var res = _context.SalesTeam.Select(s);

public class SalesTeam
{
    public string Name {get; set; }

    public Guid EmployeeId { get; set; }
    public Employee Employee { get; set; }
}
public class SalesTeamSelect
{
    public string SalesTeamName {get; set; }
    public string SalesTeamExpert {get; set; }
}