How do I make contains case-insensitive in ef core 2?

You would be better off using LIKE operator, e.g.

if (!String.IsNullOrEmpty(searchString))
{
    customers = customers.Where(x => EF.Functions.Like(x.Email, $"%{searchString}%"));
}

starting from version 2.1 of the EF Core, you can use HasConversion(). But the information in the database will be stored in lowercase:

builder.Property(it => it.Email).HasConversion(v => v.ToLowerInvariant(), v => v);

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