Dynamic column selection in LINQ

Dynamically selecting a specific column in a LINQ query can be challenging when the column is unknown at compile time. However, there is a solution that involves dynamically creating lambda expressions passed to Select.

Consider the following Data class:

public class Data
{
    public string Field1 { get; set; }
    public string Field2 { get; set; }
    public string Field3 { get; set; }
    public string Field4 { get; set; }
    public string Field5 { get; set; }
}

To dynamically select specific columns, you can use the CreateNewStatement method to create a lambda expression dynamically:

private Func CreateNewStatement(string fields)
{
    var xParameter = Expression.Parameter(typeof(Data), "o");
    var xNew = Expression.New(typeof(Data));

    var bindings = fields.Split(',')
        .Select(o => o.Trim())
        .Select(o =>
        {
            var mi = typeof(Data).GetProperty(o);
            var xOriginal = Expression.Property(xParameter, mi);
            return Expression.Bind(mi, xOriginal);
        });

    var xInit = Expression.MemberInit(xNew, bindings);
    var lambda = Expression.Lambda>(xInit, xParameter);
    return lambda.Compile();
}

This method takes a comma-separated list of fields as a string and dynamically generates a lambda expression that initializes a new Data object with the specified field.

You can then use the CreateNewStatement method to create a lambda expression and pass it to Select:

var result = list.Select(CreateNewStatement("Field1, Field2"));

This will return a sequence of Data objects with the specified field populated. Note that this technique is suitable for situations where the selected field is not known at compile time. If the selected field is static, it is more efficient to use regular lambda expressions or reflection-based methods.