ModelExtensions.cs

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using System;
using System.Linq.Expressions;
using System.Reflection;
 
using Tgstation.Server.Api.Models;
 
namespace Tgstation.Server.Host.Models
{
    static class ModelExtensions
    {
        public static TProperty Require<TModel, TProperty>(this TModel model, Expression<Func<TModel, TProperty?>> accessor)
            where TModel : EntityId
            where TProperty : struct
        {
            ArgumentNullException.ThrowIfNull(model);
            ArgumentNullException.ThrowIfNull(accessor);
 
            var memberSelectorExpression = (MemberExpression)accessor.Body;
            var property = (PropertyInfo)memberSelectorExpression.Member;
 
            var nullableValue = (TProperty?)property.GetValue(model);
            if (!nullableValue.HasValue)
                throw new InvalidOperationException($"Expected {model.GetType().Name}.{property.Name} to be set here!");
 
            return nullableValue.Value;
        }
    }
}