御用小本本

結論

var testRepository = CachedServiceProvider
    .GetRequiredService<IReadOnlyRepository<Test, Guid>>();

Expression<Func<Test, bool>> expression = x => x.Id == id;

expression = age > 18
    ? expression.And(x => x.IsAdult == ture)
    : expression.And(x => x.IsBaby == true);

var entities = await testRepository.GetListAsync(expression);

• 如果 18 歲, where 條件為 x.Id == id && x.IsAdult
• 否則 (未滿18) where 條件為 x.Id == id && x.IsBaby

P.S. 情境是直接寫條件 Linq to SQL (case then) 撈出來的資料會是錯的

GetListAsync(x => x.Id == id && age > 18 ? x.IsAdult == ture : x.IsBaby == true);

MSDN

// Add the following directive to your file:
// using System.Linq.Expressions;

// This expression perfroms a logical AND operation
// on its two arguments. Both arguments must be of the same type,
// which can be boolean or integer.
Expression andExpr = Expression.And(
    Expression.Constant(true),
    Expression.Constant(false)
);

// Print out the expression.
Console.WriteLine(andExpr.ToString());

// The following statement first creates an expression tree,
// then compiles it, and then executes it.
Console.WriteLine(Expression.Lambda<Func<bool>>(andExpr).Compile()());

// This code example produces the following output:
//
// (True And False)
// False

參照

Expression.And Method (System.Linq.Expressions) | Microsoft Learn