Program.cs

using System.Linq;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
var app = builder.Build();
app.UseSwagger();
app.UseSwaggerUI();

app.MapGet("/", () => "Hello World!");

app.MapPost("/api/claims/ingest", (List<Claim> claims) =>
{
    var validClaims = new List<Claim>();
    var flaggedClaims = new List<object>();

    foreach (var claim in claims)
    {
        var errors = ValidateClaim(claim);

        if (errors.Count == 0)
        {
            validClaims.Add(claim);
        }
        else
        {
            flaggedClaims.Add(new { Claim = claim, Errors = errors });
        }
    }

    var summary = AggregateClaims(validClaims);

    return Results.Ok(new
    {
        Summary = summary,
        FlaggedClaims = flaggedClaims
    });
});
static List<string> ValidateClaim(Claim claim)
{
    var errors = new List<string>();

    if (claim.AmountPaid < 0)
    {
        errors.Add($"AmountPaid cannot be negative (got {claim.AmountPaid}).");
    }

    if (claim.OutstandingReserve < 0)
    {
        errors.Add($"OutstandingReserve cannot be negative (got {claim.OutstandingReserve}).");
    }

    var earliestReasonableDate = new DateTime(claim.UnderwritingYear, 1, 1);
    var latestReasonableDate = new DateTime(claim.UnderwritingYear + 5, 12, 31);

    if (claim.ClaimDate < earliestReasonableDate || claim.ClaimDate > latestReasonableDate)
    {
        errors.Add(
            $"ClaimDate {claim.ClaimDate:yyyy-MM-dd} is outside a reasonable range " +
            $"for underwriting year {claim.UnderwritingYear}."
        );
    }

    return errors;
}

static object AggregateClaims(List<Claim> validClaims)
{
    var totalExposure = validClaims.Sum(c => c.AmountPaid + c.OutstandingReserve);

    var byLineOfBusiness = validClaims
        .GroupBy(c => c.LineOfBusiness)
        .Select(group => new
        {
            LineOfBusiness = group.Key,
            TotalExposure = group.Sum(c => c.AmountPaid + c.OutstandingReserve),
            ClaimCount = group.Count()
        });

    return new
    {
        TotalClaimsIngested = validClaims.Count,
        TotalFinancialExposure = totalExposure,
        ExposureByLineOfBusiness = byLineOfBusiness
    };
}

app.Run();

public record Claim(
    string ClaimId,
    string PolicyNumber,
    int UnderwritingYear,
    DateTime ClaimDate,
    decimal AmountPaid,
    decimal OutstandingReserve,
    string LineOfBusiness
);