c# 动态构建LINQ查询表达式

public Task<List<Product>> GetProductsAsync(string propertyToFilter, MathOperator mathOperator, decimal value)
{
  var query = _context.Products.AsNoTracking();

  query = propertyToFilter switch
  {
    "Amount1" when mathOperator == MathOperator.LessThanOrEqual => query.Where(x => x.Amount1 <= value),
    "Amount1" when mathOperator == MathOperator.GreaterThanOrEqual => query.Where(x => x.Amount1 >= value),

    "Amount2" when mathOperator == MathOperator.LessThanOrEqual => query.Where(x => x.Amount2 <= value),
    "Amount2" when mathOperator == MathOperator.GreaterThanOrEqual => query.Where(x => x.Amount2 >= value),

    _ => throw new ArgumentException($"不支持 {propertyToFilter} 列作为数字列查询", nameof(propertyToFilter))
  };

  return query.ToListAsync();
}

public Task<List<Product>> GetProductsAsyncV2(string propertyToFilter, MathOperator mathOperator, decimal value)
{
  var query = _context.Products.AsNoTracking();

  var paramExp = Expression.Parameter(typeof(Product));
  var memberExp = Expression.PropertyOrField(paramExp, propertyToFilter);
  var valueExp = Expression.Constant(value);
  var compareExp = mathOperator == MathOperator.LessThanOrEqual ?
    Expression.LessThanOrEqual(memberExp, valueExp) :
    Expression.GreaterThanOrEqual(memberExp, valueExp);
  var lambda = Expression.Lambda<Func<Product, bool>>(compareExp, paramExp);

  return query.Where(lambda).ToListAsync();
}

...

var toTestProduct = new Product { Amount1 = 100, Amount2 = 200 };

Func<Product, bool> func = lambda.Compile();
var result = func(toTestProduct);

Console.WriteLine($"The product's {propertyToFilter} is to {mathOperator} {value}.");

// Output: The product's Amount1 is LessThanOrEqual to 150.