string.StartsWith(char) with wild char translates to incorrect LIKE pattern on SQL Server

[colinlooser]

Bug description

Repro:

Build a DB context with UseSqlServer options having an entity with a string column.

Query using a string.StartsWith(char) filter for values starting with underscore or other LIKE wild char, e.g.:
db.MyEntities.Where(i => i.StringColumn.StartsWith('_'))...

Expect: SQL containing: WHERE [i].[StringColumn] LIKE N'\_%' ESCAPE N'\'

Actual: SQL contains: WHERE [i].[StringColumn] LIKE N'187%' ESCAPE N'\'

Impact:

The query runs, but includes/excludes the wrong rows.

Workaround:

Use the string.StartsWith(string) overload, e.g. .StartsWith("_"), rather than the char overload.

Note, the C# code analyzers encourage the use of the char overload.

Analysis:

This appears to be caused by the escape and wild char being added rather than concatenated,
e.g. 187 = '\' (92) + '_' (95), then the resulting integer being concatenated with "%".

See: src/EFCore.SqlServer/Query/Internal/SqlServerSqlTranslatingExpressionVisitor.cs

In latest main branch, same code in 2 places (for constant and parameter), switch with char s:

L581: StartsEndsWithContains.StartsWith => LikeEscapeChar + s + "%",
L765: StartsEndsWithContains.StartsWith => LikeEscapeChar + s + "%",

Note, EndsWith and Contains appear to work correctly.

Possible fix:

Change similar to how Contains works a couple of lines down, i.e. $"{LikeEscapeChar}{s}%".

Your code

// C# example mostly generated by Claude Code
// <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="10.0.11" />

using System.ComponentModel.DataAnnotations;
using System.Linq.Expressions;
using EFUnderscoreIssue;
using Microsoft.EntityFrameworkCore;

var options = new DbContextOptionsBuilder<AppDbContext>()
    .UseSqlServer("Server=.;Database=XXX;Trusted_Connection=True;TrustServerCertificate=True;")
    .Options;

using var db = new AppDbContext(options);

void Show(string label, Expression<Func<MyEntity, bool>> whereClause)
{
    IQueryable<MyEntity> query = db.MyEntities.Where(whereClause);
    string sql = query.ToQueryString();
    // Console.WriteLine(sql);

    // Pull out the DECLARE line (parameterised patterns) and the WHERE clause.
    var declare = sql.Split('\n').FirstOrDefault(l => l.StartsWith("DECLARE"))?.Trim() ?? "";
    var where = sql.Split('\n').FirstOrDefault(l => l.TrimStart().StartsWith("WHERE"))?.Trim() ?? "";

    Console.WriteLine($"{label,-32} {where}");
    if (declare.Length > 0)
    {
        Console.WriteLine($"{"",-32} {declare}");
    }
}

char underscore = '_';
string underscoreText = "_";

Console.WriteLine("=== constant argument ===");
Show("StartsWith('_')   char", i => i.StringColumn.StartsWith('_'));
Show("StartsWith(\"_\")   string", i => i.StringColumn.StartsWith("_"));
Show("StartsWith('%')   char", i => i.StringColumn.StartsWith('%'));
Show("StartsWith('[')   char", i => i.StringColumn.StartsWith('['));
Show("StartsWith('A')   char", i => i.StringColumn.StartsWith('A'));
Show("EndsWith('_')     char", i => i.StringColumn.EndsWith('_'));
Show("Contains('_')     char", i => i.StringColumn.Contains('_'));

Console.WriteLine();
Console.WriteLine("=== captured variable (parameterised) ===");
Show("StartsWith(char)  '_'", i => i.StringColumn.StartsWith(underscore));
Show("StartsWith(string) \"_\"", i => i.StringColumn.StartsWith(underscoreText));
Show("EndsWith(char)    '_'", i => i.StringColumn.EndsWith(underscore));

namespace EFUnderscoreIssue
{
    public class MyEntity
    {
        [Key]
        public int Id { get; set; }
        public string StringColumn { get; set; } = string.Empty;
    }

    public class AppDbContext(DbContextOptions<AppDbContext> options) : DbContext(options)
    {
        public DbSet<MyEntity> MyEntities { get; set; }
    }
}

Stack traces

Verbose output

=== constant argument ===
StartsWith('_')   char           WHERE [m].[StringColumn] LIKE N'187%' ESCAPE N'\'
StartsWith("_")   string         WHERE [m].[StringColumn] LIKE N'\_%' ESCAPE N'\'
StartsWith('%')   char           WHERE [m].[StringColumn] LIKE N'129%' ESCAPE N'\'
StartsWith('[')   char           WHERE [m].[StringColumn] LIKE N'183%' ESCAPE N'\'
StartsWith('A')   char           WHERE [m].[StringColumn] LIKE N'A%'
EndsWith('_')     char           WHERE [m].[StringColumn] LIKE N'%\_' ESCAPE N'\'
Contains('_')     char           WHERE [m].[StringColumn] LIKE N'%\_%' ESCAPE N'\'

=== captured variable (parameterised) ===
StartsWith(char)  '_'            WHERE [m].[StringColumn] LIKE @underscore_startswith ESCAPE N'\'
                                 DECLARE @underscore_startswith nvarchar(4000) = N'187%';
StartsWith(string) "_"           WHERE [m].[StringColumn] LIKE @underscoreText_startswith ESCAPE N'\'
                                 DECLARE @underscoreText_startswith nvarchar(4000) = N'\_%';
EndsWith(char)    '_'            WHERE [m].[StringColumn] LIKE @underscore_endswith ESCAPE N'\'
                                 DECLARE @underscore_endswith nvarchar(4000) = N'%\_';

EF Core version

10.0.11

Database provider

Microsoft.EntityFrameworkCore.SqlServer

Target framework

.NET 10

Operating system

Windows 11

IDE

Visual Studio 2026 18.6.2