https://medium.com/@buenojeferson/e566f929846b

classe.cs

class Compra
{
    public string Cliente { get; set; }
    public decimal Valor { get; set; }
    public DateTime Data { get; set; }

    public override string ToString() => $"{Cliente}, com valor de {Valor:n2} em {Data:dd/MM}";
}

dados.cs

IReadOnlyCollection<Compra> Compras => new []
{
    new Compra { Valor =  100, Data = new DateTime(2019, 01, 19), Cliente = "Finn" },
    new Compra { Valor =  200, Data = new DateTime(2019, 02, 05), Cliente = "Jake" },
    new Compra { Valor = 1350, Data = new DateTime(2019, 02, 07), Cliente = "BMO" },
    new Compra { Valor =  900, Data = new DateTime(2019, 01, 21), Cliente = "Finn" },
    new Compra { Valor =  150, Data = new DateTime(2019, 02, 05), Cliente = "Jake" },
    new Compra { Valor = 1500, Data = new DateTime(2019, 02, 09), Cliente = "Finn" },
    new Compra { Valor =  300, Data = new DateTime(2019, 03, 10), Cliente = "Marceline" },
    new Compra { Valor = 1200, Data = new DateTime(2019, 03, 15), Cliente = "Marceline" },
    new Compra { Valor = 1900, Data = new DateTime(2019, 01, 10), Cliente = "Marceline" },
    new Compra { Valor = 2500, Data = new DateTime(2019, 03, 18), Cliente = "Finn" }
};