IsNumeric in EF Core

This has now been added to EF Core and it looks like it is part of EF Core 6.0 - https://github.com/dotnet/efcore/issues/23114

You could use Raw SQL Queries

Security Note: Always use parameterization for raw SQL queries

Given this dataset/result:

The entity class for EF Core: Sandbox.cs

    public class Sandbox
    {
        public int SandboxId { get; set; }
        public string StringOrNumber { get; set; }
    }

Create a DbSet in your context:

    public DbSet<Sandbox> Sandboxes { get; set; }

Mark as no key:

    modelBuilder.Entity<Sandbox>().HasNoKey();

Test:

    public async Task<IList<Sandbox>> GetNumberList()
    {
        // 1 = number, 0 = string
        var isNumeric = new SqlParameter("isNumeric", 1);

        var listOfNumbers = await _context.Sandboxes
        .FromSqlRaw("SELECT SandboxId, StringOrNumber FROM dbo.Sandbox WHERE ISNUMERIC(StringOrNumber) = @isNumeric", isNumeric)
        .ToListAsync();

        return listOfNumbers;
    }

An alternative to FromSqlRaw is FromSqlInterpolated.

    var listOfNumbers = await _context.Sandboxes
    .FromSqlInterpolated($"SELECT SandboxId, StringOrNumber FROM dbo.Sandbox WHERE ISNUMERIC(StringOrNumber) = {isNumeric}")
    .ToListAsync();

ISNUMERIC function is part of EF Core 6.0 in EF.Functions